Updated version 0.9.0 from 'upstream/0.9.0'

with Debian dir 04f2bb3e026e2a983dcac5cdcc35c9dd8f6b91ab

1 files changed, 1451 insertions, 0 deletions

diff --git a/raphodo/devices.py b/raphodo/devices.py
new file mode 100644
index 0000000..a2597fe
--- /dev/null
+++ b/raphodo/devices.py
@@ -0,0 +1,1451 @@
+# Copyright (C) 2015-2017 Damon Lynch <damonlynch@gmail.com>
+
+# This file is part of Rapid Photo Downloader.
+#
+# Rapid Photo Downloader is free software: you can redistribute it and/or
+# modify it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# Rapid Photo Downloader is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with Rapid Photo Downloader.  If not,
+# see <http://www.gnu.org/licenses/>.
+
+"""
+Handle Devices and Device Collections.
+
+In Rapid Photo Downloader, "Device" has two meanings, depending on the
+context:
+1. In the GUI, a Device is a camera or a volume (external drive)
+2. In code, a Device is one of a camera, volume, or path
+"""
+
+__author__ = 'Damon Lynch'
+__copyright__ = "Copyright 2015-2017, Damon Lynch"
+
+import sys
+import shutil
+import os
+import logging
+import itertools
+from collections import namedtuple, Counter, defaultdict
+from typing import Tuple, List, Optional, Set, Dict, Union, DefaultDict
+
+from gettext import gettext as _
+
+from PyQt5.QtCore import QStorageInfo, QSize
+from PyQt5.QtWidgets import QFileIconProvider
+from PyQt5.QtGui import QIcon, QPixmap
+
+import raphodo.qrc_resources as qrc_resources
+from raphodo.constants import (
+    DeviceType, BackupLocationType, FileType, DeviceState, DownloadStatus, ExifSource,
+    DownloadingFileTypes, BackupFailureType
+)
+from raphodo.rpdfile import FileTypeCounter, FileSizeSum
+from raphodo.storage import (
+    StorageSpace, udev_attributes, UdevAttr, get_path_display_name, validate_download_folder,
+    ValidatedFolder, CameraDetails, get_uri, fs_device_details
+)
+from raphodo.camera import generate_devname
+from raphodo.utilities import (
+    number, make_internationalized_list, stdchannel_redirected, same_device
+)
+import raphodo.metadataphoto as metadataphoto
+from raphodo.rpdfile import Photo, Video
+import raphodo.exiftool as exiftool
+from raphodo.problemnotification import FsMetadataWriteProblem
+
+display_devices = (DeviceType.volume, DeviceType.camera)
+
+
+class Device:
+    r"""
+    Representation of a camera, or a device, or a path.
+    Files will be downloaded from it.
+
+    To run the doctests, ensure at least one camera is plugged in
+    but not mounted!
+
+    >>> d = Device()
+    >>> d.set_download_from_volume('/media/damon/EOS_DIGITAL', 'EOS_DIGITAL')
+    >>> d
+    'EOS_DIGITAL':'/media/damon/EOS_DIGITAL'
+    >>> str(d)
+    '/media/damon/EOS_DIGITAL (EOS_DIGITAL)'
+    >>> d.display_name
+    'EOS_DIGITAL'
+    >>> d.camera_model
+    >>> d.camera_port
+
+    >>> import gphoto2 as gp
+    >>> gp_context = gp.Context()
+    >>> cameras = gp_context.camera_autodetect()
+    >>> c = Device()
+    >>> for model, port in cameras:
+    ...     c.set_download_from_camera(model, port)
+    ...     isinstance(c.display_name, str)
+    True
+    >>> e = Device()
+    >>> e.set_download_from_volume('/media/damon/EOS_DIGITAL', 'EOS_DIGITAL')
+    >>> e == d
+    True
+    >>> e != c
+    True
+    >>> c == d
+    False
+    >>> c != d
+    True
+    """
+    def __init__(self):
+        self.clear()
+
+    def clear(self):
+        self.camera_model = None  # type: str
+        self.camera_port = None  # type: str
+        # Assume an MTP device is likely a smart phone or tablet
+        self.is_mtp_device = False
+        self.udev_name = None  # type: str
+        self.storage_space = []  # type: List[StorageSpace]
+        # Name of storage on a camera
+        self.storage_descriptions = []  # type: List[str]
+
+        self.path = None  # type: str
+        self.display_name = None  # type: str
+        self.have_optimal_display_name = False
+        self.device_type = None  # type: DeviceType
+        self.icon_name = None  # type: str
+        self.can_eject = None  # type: bool
+        self.photo_cache_dir = None  # type: str
+        self.video_cache_dir = None  # type: str
+        self.file_size_sum = FileSizeSum()
+        self.file_type_counter = FileTypeCounter()
+        self.download_statuses = set()  # type: Set[DownloadStatus]
+        self._uri = ''
+
+    def __repr__(self):
+        if self.device_type == DeviceType.camera:
+            return "%r:%r" % (self.camera_model, self.camera_port)
+        elif self.device_type == DeviceType.volume:
+            return "%r:%r" % (self.display_name, self.path)
+        else:
+            return "%r" % self.path
+
+    def __str__(self):
+        if self.device_type == DeviceType.camera:
+            return '{} on port {}. Udev: {}; Display name: {} (optimal: {}); MTP: {}'.format(
+                              self.camera_model, self.camera_port, self.udev_name,
+                              self.display_name, self.have_optimal_display_name, self.is_mtp_device)
+        elif self.device_type == DeviceType.volume:
+            if self.path != self.display_name:
+                return "%s (%s)" % (self.path, self.display_name)
+            else:
+                return "%s" % (self.path)
+        else:
+            return "%s" % (self.path)
+
+    def __eq__(self, other):
+        for attr in ('device_type', 'camera_model', 'camera_port', 'path'):
+            if getattr(self, attr) != getattr(other, attr):
+                return False
+        return True
+
+    def __hash__(self):
+        return hash((self.device_type, self.camera_model, self.camera_port, self.path))
+
+    def __ne__(self, other):
+        return not self.__eq__(other)
+
+    def _get_valid_icon_name(self, possible_names):
+        if possible_names is not None:
+            for icon_name in possible_names:
+                if QIcon.hasThemeIcon(icon_name):
+                    return icon_name
+        return None
+
+    @property
+    def uri(self) -> str:
+        if self._uri:
+            return self._uri
+
+        if self.device_type == DeviceType.camera:
+            if self.storage_descriptions:
+                storage_desc = self.storage_descriptions[0]
+            else:
+                storage_desc = ''
+            camera_details = CameraDetails(
+                model=self.camera_model, port=self.camera_port, display_name=self.display_name,
+                is_mtp=self.is_mtp_device, storage_desc=storage_desc
+            )
+            self._uri = get_uri(camera_details=camera_details)
+        else:
+            self._uri = get_uri(path=self.path)
+
+        return self._uri
+
+    def set_download_from_camera(self, camera_model: str, camera_port: str) -> None:
+        self.clear()
+        self.device_type = DeviceType.camera
+        self.camera_model = camera_model
+        # Set default display name, for when all else fails.
+        # Try to override this value below
+        self.display_name = camera_model
+        self.camera_port = camera_port
+        self.icon_name = self._get_valid_icon_name(('camera-photo', 'camera'))
+
+        # Assign default udev name if cannot determine from udev itself
+        self.udev_name = camera_model
+
+        devname = generate_devname(camera_port)
+        if devname is not None:
+            udev_attr = udev_attributes(devname)
+            if udev_attr is not None:
+                self.is_mtp_device = udev_attr.is_mtp_device
+                self.udev_name = udev_attr.model
+                self.display_name = udev_attr.model
+        else:
+            logging.error("Could not determine udev values for %s %s",
+                          self.camera_model, camera_port)
+
+    def update_camera_attributes(self, display_name: str,
+                                 storage_space: List[StorageSpace],
+                                 storage_descriptions: List[str]) -> None:
+        self.display_name = display_name
+        self.have_optimal_display_name = True
+        self.storage_space = storage_space
+        self.storage_descriptions = storage_descriptions
+
+    def set_download_from_volume(self, path: str, display_name: str,
+                                 icon_names=None, can_eject=None,
+                                 mount: QStorageInfo=None) -> None:
+        self.clear()
+        self.device_type = DeviceType.volume
+        self.path = path
+        self.icon_name = self._get_valid_icon_name(icon_names)
+        if not display_name.find(os.sep) >= 0:
+            self.display_name = display_name
+        else:
+            self.display_name = os.path.basename(display_name)
+        self.have_optimal_display_name = True
+        self.can_eject = can_eject
+        if not mount:
+            mount = QStorageInfo(path)
+        self.storage_space.append(StorageSpace(
+                        bytes_free=mount.bytesAvailable(),
+                        bytes_total=mount.bytesTotal(),
+                        path=path))
+
+    def set_download_from_path(self, path: str) -> None:
+        self.clear()
+        self.device_type = DeviceType.path
+        self.path = path
+        if path.endswith(os.sep):
+            path = path[:-1]
+        display_name = os.path.basename(path)
+        if display_name:
+            self.display_name = display_name
+            self.have_optimal_display_name = True
+        else:
+            self.display_name = path
+        # the next value is almost certainly ("folder",), but I guess it's
+        # better to generate it from code
+        self.icon_name = ('{}'.format(QFileIconProvider().icon(
+            QFileIconProvider.Folder).name()))
+        mount = QStorageInfo(path)
+        self.storage_space.append(StorageSpace(
+                        bytes_free=mount.bytesAvailable(),
+                        bytes_total=mount.bytesTotal(),
+                        path=path))
+
+    def get_storage_space(self, index: int=0) -> StorageSpace:
+        """
+        Convenience function to retrieve information about bytes
+        free and bytes total (capacity of the media). Almost all
+        devices have only one storage media, but some cameras have
+        more than one
+        :param index: the storage media to get the values from
+        :return: tuple of bytes free and bytes total
+        """
+        return self.storage_space[index]
+
+    def name(self) -> str:
+        """
+        Get the name of the device, suitable to be displayed to the
+        user. If the device is a path, return the path name
+        :return  str containg the name
+        """
+        if self.device_type == DeviceType.camera:
+            return self.display_name
+        elif self.device_type == DeviceType.volume:
+            return self.display_name
+        else:
+            return self.path
+
+    def get_icon(self) -> QIcon:
+        """Return icon for the device."""
+
+        if self.device_type == DeviceType.volume:
+            return QIcon(':icons/drive-removable-media.svg')
+        elif self.device_type == DeviceType.path:
+            return QIcon(':/icons/folder.svg')
+        else:
+            assert self.device_type == DeviceType.camera
+            if self.is_mtp_device:
+                if self.camera_model.lower().find('tablet') >= 0:
+                    #TODO use tablet icon
+                    pass
+                return QIcon(':icons/smartphone.svg')
+            return QIcon(':/icons/camera.svg')
+
+    def get_pixmap(self, size: QSize=QSize(30, 30)) -> QPixmap:
+        icon = self.get_icon()
+        return icon.pixmap(size)
+
+    def _delete_cache_dir(self, cache_dir) -> None:
+        if cache_dir:
+            if os.path.isdir(cache_dir):
+                assert cache_dir != os.path.expanduser('~')
+                try:
+                    shutil.rmtree(cache_dir, ignore_errors=True)
+                except:
+                    logging.error("Unknown error deleting cache directory %s", cache_dir)
+
+    def delete_cache_dirs(self) -> None:
+        self._delete_cache_dir(self.photo_cache_dir)
+        self._delete_cache_dir(self.video_cache_dir)
+
+
+
+class DeviceCollection:
+    """
+    Maintain collection of devices that are being scanned, where a
+    device is of type Device.
+
+    When a device is added, a scan_id is generated and returned.
+
+    >>> d = Device()
+    >>> d.set_download_from_volume('/media/damon/EOS_DIGITAL', 'EOS_DIGITAL')
+    >>> c = Device()
+    >>> c.set_download_from_camera('Canon EOS 1D X', 'usb:001,002')
+    >>> e = Device()
+    >>> e.set_download_from_volume('/media/damon/EOS_DIGITAL', 'EOS_DIGITAL')
+    >>> dc = DeviceCollection()
+    >>> d_scan_id = dc.add_device(d)
+    >>> d_scan_id
+    0
+    >>> d_scan_id in dc
+    True
+    >>> dc.known_path(d.path, DeviceType.volume)
+    True
+    >>> dc.known_path(d.path)
+    True
+    >>> dc[d_scan_id] == d
+    True
+    >>> len(dc.volumes_and_cameras)
+    1
+    >>> len(dc.this_computer)
+    0
+    >>> dc.known_path('/root', DeviceType.path)
+    False
+    >>> dc.known_path('/root')
+    False
+    >>> c_scan_id = dc.add_device(c)
+    >>> c_scan_id
+    1
+    >>> len(dc)
+    2
+    >>> len(dc.volumes_and_cameras)
+    2
+    >>> len(dc.this_computer)
+    0
+    >>> dc[d_scan_id] == dc[c_scan_id]
+    False
+    >>> dc.known_camera('Canon EOS 1D X', 'usb:001,002')
+    True
+    >>> dc.known_camera('Canon EOS 1D X', 'usb:001,003')
+    False
+    >>> dc.delete_device(c)
+    True
+    >>> len(dc.cameras)
+    0
+    >>> len(dc.volumes_and_cameras)
+    1
+    >>> len(dc.this_computer)
+    0
+    >>> dc.known_camera('Canon EOS 1D X', 'usb:001,002')
+    False
+    >>> len(dc)
+    1
+    >>> dc.known_device(e)
+    True
+    >>> del dc[d_scan_id]
+    >>> len(dc)
+    0
+    >>> len(dc.volumes_and_cameras)
+    0
+    >>> len(dc.this_computer)
+    0
+    >>> dc.delete_device(e)
+    False
+    """
+    def __init__(self, exiftool_process: Optional[exiftool.ExifTool]=None,
+                 rapidApp=None) -> None:
+
+        self.rapidApp = rapidApp
+
+        self.devices = {}  # type: Dict[int, Device]
+        # port: model
+        self.cameras = {}  # type: Dict[str, str]
+
+        # Used to assign scan ids
+        self.scan_counter = 0  # type: int
+
+        # scan_id: DeviceState
+        self.device_state = {}  # type: Dict[int, DeviceState]
+
+        # Track which devices are being scanned, by scan_id
+        self.scanning = set()  # type: Set[int]
+        # Track which downloads are running, by scan_id
+
+        self.downloading = set()  # type: Set[int]
+        # Track which devices have been downloaded from during one
+        # download, by display name. Must do it by display name
+        # because some devices could be removed before all devices
+        # have been downloaded from.
+        self.have_downloaded_from = set()  # type: Set[str]
+
+        # Track which devices are thumbnailing, by scan_id
+        self.thumbnailing = set()  # type: Set[int]
+
+        # Track the unmounting of unscanned cameras by port and model
+        # port: model
+        self.cameras_to_gvfs_unmount_for_scan = {}  # type: Dict[str, str]
+
+        # Which scanned cameras need to be unmounted for a download to start, by scan_id
+        self.cameras_to_gvfs_unmount_for_download = set()  # type: Set[int]
+        self.cameras_to_stop_thumbnailing = set()
+
+        # Automatically detected devices where the user has explicitly said to ignore it
+        # port: model
+        self.ignored_cameras = {}  # type: Dict[str, str]
+        # List[path]
+        self.ignored_volumes = []  # type: List[str]
+
+        # Devices that were set to autodownload while the program
+        # is in a paused state
+        self.queued_to_download = set()  # type: Set[int]
+
+        self.volumes_and_cameras = set()  # type: Set[int]
+        self.this_computer = set()  # type: Set[int]
+
+        # List of devices that were detected at program startup
+        # scan_id
+        self.startup_devices = []  # type: List[int]
+
+        # Sample exif bytes of photo on most recent device scanned
+        self._sample_photo = None  # type: Optional[Photo]
+        self._sample_video = None  # type: Optional[Video]
+        self._sample_videos_complete_files = []  # type: List[str]
+        self.exiftool_process = exiftool_process
+
+        self._map_set = {DeviceType.path: self.this_computer,
+                         DeviceType.camera: self.volumes_and_cameras,
+                         DeviceType.volume: self.volumes_and_cameras}
+        self._map_plural_types = {DeviceType.camera: _('Cameras'),
+                                  DeviceType.volume: _('Devices')}
+
+    def download_start_blocked(self) -> bool:
+        """
+        Determine if a camera needs to be unmounted or thumbnailing needs to be
+        terminated for a camera in order for a download to proceed
+        :return: True if so, else False
+        """
+
+        if len(self.cameras_to_gvfs_unmount_for_download) > 0 and len(
+                self.cameras_to_stop_thumbnailing):
+            logging.debug("Download is blocked because %s camera(s) are being unmounted from GVFS "
+                          "and %s camera(s) are having their thumbnailing terminated",
+                          len(self.cameras_to_gvfs_unmount_for_download),
+                          len(self.cameras_to_stop_thumbnailing))
+        elif len(self.cameras_to_gvfs_unmount_for_download) > 0:
+            logging.debug("Download is blocked because %s camera(s) are being unmounted from GVFS",
+                          len(self.cameras_to_gvfs_unmount_for_download))
+        elif len(self.cameras_to_stop_thumbnailing) > 0:
+            logging.debug("Download is blocked because %s camera(s) are having their thumbnailing "
+                          "terminated", len(self.cameras_to_stop_thumbnailing))
+
+        return len(self.cameras_to_gvfs_unmount_for_download) > 0 or len(
+                    self.cameras_to_stop_thumbnailing) > 0
+
+    def logState(self) -> None:
+        logging.debug("-- Device Collection --")
+        logging.debug('%s devices: %s volumes/cameras (%s cameras), %s this computer',
+                      len(self.devices), len(self.volumes_and_cameras), len(self.cameras),
+                      len(self.this_computer))
+        logging.debug("Device states: %s", ', '.join(
+            '%s: %s' % (self[scan_id].display_name, self.device_state[scan_id].name)
+            for scan_id in self.device_state))
+        if len(self.scanning):
+            scanning = ('%s' % ', '.join(self[scan_id].display_name for scan_id in self.scanning))
+            logging.debug("Scanning: %s", scanning)
+        else:
+            logging.debug("No devices scanning")
+        if len(self.downloading):
+            downloading = ('%s' % ', '.join(self[scan_id].display_name
+                                         for scan_id in self.downloading))
+            logging.debug("Downloading: %s", downloading)
+        else:
+            logging.debug("No devices downloading")
+        if len(self.thumbnailing):
+            thumbnailing = ('%s' % ', '.join(self[scan_id].display_name
+                                         for scan_id in self.thumbnailing))
+            logging.debug("Thumbnailing: %s", thumbnailing)
+        else:
+            logging.debug("No devices thumbnailing")
+
+    def add_device(self, device: Device, on_startup: bool=False) -> int:
+        """
+        Add a new device to the device collection
+        :param device: device to add
+        :param on_startup: if True, the device is being added during
+         the program's startup phase
+        :return: the scan id assigned to the device
+        """
+
+        scan_id = self.scan_counter
+        self.scan_counter += 1
+        self.devices[scan_id] = device
+        self.device_state[scan_id] = DeviceState.pre_scan
+        if on_startup:
+            self.startup_devices.append(scan_id)
+        if device.camera_port:
+            port = device.camera_port
+            assert port not in self.cameras
+            self.cameras[port] = device.camera_model
+        if device.device_type in display_devices:
+            self.volumes_and_cameras.add(scan_id)
+        else:
+            self.this_computer.add(scan_id)
+        return scan_id
+
+    def set_device_state(self, scan_id: int, state: DeviceState) -> None:
+        logging.debug("Setting device state for %s to %s",
+                      self.devices[scan_id].display_name, state.name)
+        self.device_state[scan_id] = state
+        if state == DeviceState.scanning:
+            self.scanning.add(scan_id)
+        elif state == DeviceState.downloading:
+            self.downloading.add(scan_id)
+            self.have_downloaded_from.add(self.devices[scan_id].display_name)
+        elif state == DeviceState.thumbnailing:
+            self.thumbnailing.add(scan_id)
+
+        if state != DeviceState.scanning and scan_id in self.scanning:
+            self.scanning.remove(scan_id)
+        if state != DeviceState.downloading and scan_id in self.downloading:
+            self.downloading.remove(scan_id)
+        if state != DeviceState.thumbnailing and scan_id in self.thumbnailing:
+            self.thumbnailing.remove(scan_id)
+
+    def ignore_device(self, scan_id: int) -> None:
+        """
+        For the remainder of this program's instantiation, don't
+        automatically detect this device.
+
+        A limitation of this is that when a camera is physically removed
+        and plugged in again, it gets a new port. In which casae it's a
+        "different" device.
+
+        :param scan_id: scan id of the device to ignore
+        """
+
+        device = self.devices[scan_id]
+        if device.device_type == DeviceType.camera:
+            logging.debug("Marking camera %s on port %s as explicitly removed. Will ignore it "
+                          "until program exit.", device.camera_model, device.camera_port)
+            self.ignored_cameras[device.camera_port] = device.camera_model
+        elif device.device_type == DeviceType.volume:
+            logging.debug("Marking volume %s as explicitly removed. Will ignore it "
+                          "until program exit.", device.path)
+            self.ignored_volumes.append(device.path)
+        else:
+            logging.error("Device collection unexpectedly received path to ignore: ignoring")
+
+    def user_marked_camera_as_ignored(self, model: str, port: str) -> bool:
+        """
+        Check if camera is in set of devices to ignore because they were explicitly
+        removed by the user
+
+        :param model: camera model
+        :param port:  camera port
+        :return: return True if camera is in set of devices to ignore
+        """
+
+        if port in self.ignored_cameras:
+            return self.ignored_cameras[port] == model
+        return False
+
+    def user_marked_volume_as_ignored(self, path: str) -> bool:
+        """
+        Check if volume's path is in list of devices to ignore because they were explicitly
+        removed by the user
+
+        :param: path: the device's path
+        :return: return True if camera is in set of devices to ignore
+        """
+
+        return path in self.ignored_volumes
+
+    def known_camera(self, model: str, port: str) -> bool:
+        """
+        Check if the camera is already in the list of devices
+        :param model: camera model as specified by libgohoto2
+        :param port: camera port as specified by libgohoto2
+        :return: True if this camera is already being processed, else False
+        """
+        if port in self.cameras:
+            assert self.cameras[port] == model
+            return True
+        return False
+
+    def known_path(self, path: str, device_type: Optional[DeviceType]=None) -> bool:
+        """
+        Check if the path is already in the list of devices
+        :param path: path to check
+        :return: True if the path is already being processed, else False
+        """
+        for scan_id in self.devices:
+            device = self.devices[scan_id]  # type: Device
+            if device.path == path:
+                if device_type is None or device.device_type == device_type:
+                    return True
+        return False
+
+    def known_device(self, device: Device) -> bool:
+        return device in list(self.devices.values())
+
+    def scan_id_from_path(self, path: str, device_type: Optional[DeviceType]=None) -> Optional[int]:
+        for scan_id, device in self.devices.items():
+            if device.path == path:
+                if device_type is None or device.device_type == device_type:
+                    return scan_id
+        return None
+
+    def scan_id_from_camera_model_port(self, model: str, port: str) -> Optional[int]:
+        """
+
+        :param model: model name of camera being searched for
+        :param port: port of camera being searched for
+        :return: scan id of camera if known, else None
+        """
+
+        for scan_id, device in self.devices.items():
+            if (device.device_type == DeviceType.camera and device.camera_model == model and
+                    device.camera_port == port):
+                return scan_id
+        return None
+
+    def delete_device(self, device: Device) -> bool:
+        """
+        Delete the device from the collection.
+        :param device: the device to delete
+        :return: True if device was deleted, else return False
+        """
+        for scan_id in self.devices:
+            if self.devices[scan_id] == device:
+                del self[scan_id]
+                return True
+        return False
+
+    def delete_cache_dirs_and_sample_video(self) -> None:
+        """
+        Delete all Download Caches and their contents any devices might
+        have, as well as any sample video.
+        """
+        for device in self.devices.values():
+            device.delete_cache_dirs()
+        self._delete_sample_video(at_program_close=True)
+
+    def _delete_sample_video(self, at_program_close: bool) -> None:
+        """
+        Delete sample video that is used for metadata extraction
+        to provide example for file renaming.
+
+        Does not delete
+
+        :param at_program_close: if True, the program is exiting
+        """
+
+        if (self._sample_video is not None and
+                self._sample_video.temp_sample_full_file_name is not None and
+                self._sample_video.from_camera):
+            try:
+                assert self._sample_video.temp_sample_full_file_name
+            except:
+                logging.error("Expected sample file name in sample video")
+            else:
+                if os.path.isfile(self._sample_video.temp_sample_full_file_name):
+                    logging.info("Removing temporary sample video %s",
+                                 self._sample_video.temp_sample_full_file_name)
+                    try:
+                        os.remove(self._sample_video.temp_sample_full_file_name)
+                    except Exception:
+                        logging.exception("Error removing temporary sample video file %s",
+                                          self._sample_video.temp_sample_full_file_name)
+
+        if at_program_close and self._sample_videos_complete_files:
+            remaining_videos = (video for video in self._sample_videos_complete_files
+                                if os.path.isfile(video))
+            for video in remaining_videos:
+                logging.info("Removing temporary sample video %s", video)
+                try:
+                    os.remove(video)
+                except Exception:
+                    logging.exception("Error removing temporary sample video file %s", video)
+
+    def map_set(self, device: Device) -> Set:
+        return self._map_set[device.device_type]
+
+    def downloading_from(self) -> str:
+        """
+        :return: string showing which devices are being downloaded from
+        """
+
+        display_names = [self.devices[scan_id].display_name for scan_id in self.downloading]
+        return _('Downloading from %(device_names)s') % dict(
+            device_names=make_internationalized_list(display_names))
+
+    def reset_and_return_have_downloaded_from(self) -> str:
+        """
+        Reset the set of devices that have been downloaded from,
+        and return the string that
+        :return: string showing which devices have been downloaded from
+         during this download
+        """
+        display_names = make_internationalized_list(list(self.have_downloaded_from))
+        self.have_downloaded_from = set()  # type: Set[str]
+        return display_names
+
+    def __delitem__(self, scan_id: int):
+        d = self.devices[scan_id]  # type: Device
+        if d.device_type == DeviceType.camera:
+            del self.cameras[d.camera_port]
+            if d.camera_port in self.cameras_to_gvfs_unmount_for_scan:
+                del self.cameras_to_gvfs_unmount_for_scan[d.camera_port]
+
+        self.map_set(d).remove(scan_id)
+        d.delete_cache_dirs()
+        del self.devices[scan_id]
+        if scan_id in self.scanning:
+            self.scanning.remove(scan_id)
+        if scan_id in self.downloading:
+            self.downloading.remove(scan_id)
+        if scan_id in self.queued_to_download:
+            self.queued_to_download.remove(scan_id)
+        if scan_id in self.thumbnailing:
+            self.thumbnailing.remove(scan_id)
+        if scan_id in self.cameras_to_gvfs_unmount_for_download:
+            self.cameras_to_gvfs_unmount_for_download.remove(scan_id)
+        if scan_id in self.cameras_to_stop_thumbnailing:
+            self.cameras_to_stop_thumbnailing.remove(scan_id)
+        if scan_id in self.this_computer:
+            self.this_computer.remove(scan_id)
+        if scan_id in self.volumes_and_cameras:
+            self.volumes_and_cameras.remove(scan_id)
+        del self.device_state[scan_id]
+
+    def __getitem__(self, scan_id: int) -> Device:
+        return self.devices[scan_id]
+
+    def __len__(self) -> int:
+        return len(self.devices)
+
+    def __contains__(self, scan_id: int) -> bool:
+        return scan_id in self.devices
+
+    def __iter__(self):
+        return iter(self.devices)
+
+    def _mixed_devices(self, device_type_text: str) -> str:
+        try:
+            text_number = number(len(self.volumes_and_cameras)).number.capitalize()
+        except KeyError:
+            text_number = len(self.volumes_and_cameras)
+        # Translators: e.g. Three Devices
+        return _('%(no_devices)s %(device_type)s') % dict(
+            no_devices=text_number, device_type=device_type_text)
+
+    def _update_sample_file(self, file_type: FileType) -> None:
+
+        if file_type == FileType.photo:
+            assert self._sample_photo.file_type == FileType.photo
+            full_file_name = self._sample_photo.full_file_name
+            rpd_file = self._sample_photo
+        else:
+            assert self._sample_video.file_type == FileType.video
+            full_file_name = self._sample_video_full_file_name()
+            rpd_file = self._sample_video
+
+        if not os.path.isfile(full_file_name):
+            # file no longer exists - it may have been downloaded or deleted
+            # attempt to find an appropriate file from the in memory sql database of displayed
+            # files
+            scan_id = rpd_file.scan_id
+            rpd_file = self.rapidApp.thumbnailModel.getSampleFile(scan_id=scan_id,
+                                      device_type=self[scan_id].device_type, file_type=file_type)
+            if rpd_file is None:
+                logging.debug('Failed to set new sample %s because suitable sample does not'
+                              'exist', file_type.name)
+            else:
+                sample_full_file_name = rpd_file.get_current_full_file_name()
+                if file_type == FileType.photo:
+                    logging.debug('Updated sample photo with %s', sample_full_file_name)
+                    self.sample_photo = rpd_file
+                else:
+                    logging.debug('Updated sample video with %s', sample_full_file_name)
+                    self.sample_video = rpd_file
+
+    @property
+    def sample_photo(self) -> Optional[Photo]:
+        if self._sample_photo is None:
+            return None
+
+        # does the photo still exist?
+        if self._sample_photo.exif_source == ExifSource.actual_file:
+            self._update_sample_file(file_type=FileType.photo)
+
+        if self._sample_photo.metadata is None and not self._sample_photo.metadata_failure:
+            with stdchannel_redirected(sys.stderr, os.devnull):
+                if self._sample_photo.exif_source == ExifSource.raw_bytes:
+                    self._sample_photo.load_metadata(
+                        raw_bytes=bytearray(self._sample_photo.raw_exif_bytes))
+                elif self._sample_photo.exif_source == ExifSource.app1_segment:
+                    self._sample_photo.load_metadata(
+                        app1_segment=bytearray(self._sample_photo.raw_exif_bytes))
+                else:
+                    assert self._sample_photo.exif_source == ExifSource.actual_file
+                    full_file_name = self._sample_photo.get_current_full_file_name()
+                    self._sample_photo.load_metadata(full_file_name=full_file_name,
+                                                     et_process=self.exiftool_process)
+        return self._sample_photo
+
+    @sample_photo.setter
+    def sample_photo(self, photo: Photo) -> None:
+        self._sample_photo = photo
+
+    def _sample_video_full_file_name(self) -> str:
+        """
+        Sample videos can be either excerpts of a video from a camera or
+        actual videos already on the file system.
+
+        :return: the full path and filename of the sample video regardless
+         of which type it is
+        """
+
+        if (self._sample_video is not None and
+                self._sample_video.temp_sample_full_file_name is not None):
+            full_file_name = self._sample_video.temp_sample_full_file_name
+        else:
+            full_file_name = self._sample_video.get_current_full_file_name()
+        return full_file_name
+        
+    @property
+    def sample_video(self) -> Optional[Video]:
+        if self._sample_video is None:
+            return None
+
+        self._update_sample_file(file_type=FileType.video)
+
+        if self._sample_video.metadata is None and not self._sample_video.metadata_failure:
+
+            try:
+                assert self._sample_video.temp_sample_full_file_name or os.path.isfile(
+                    self._sample_video.full_file_name)
+
+                full_file_name = self._sample_video_full_file_name()
+
+                self._sample_video.load_metadata(
+                    full_file_name=full_file_name,
+                    et_process=self.exiftool_process)
+                if self._sample_video.metadata_failure:
+                    logging.error("Failed to load sample video metadata")
+            except AssertionError:
+                logging.error("Expected sample file name in sample video")
+            except:
+                logging.error("Exception while attempting to load sample video metadata")
+        return self._sample_video
+
+    @sample_video.setter
+    def sample_video(self, video: Video) -> None:
+        if self._sample_video is not None and self._sample_video.temp_sample_is_complete_file:
+            # Don't delete this fully downloaded file, as it might be downloaded by the user,
+            # in which case it's already been recorded as a RPDFile.cache_full_file_name
+            # Instead add it to a list of files to possibly expunge at program exit
+            logging.debug("Adding %s to list of complete sample video files to potentially delete "
+                          "at program exit", self._sample_video.temp_sample_full_file_name)
+            self._sample_videos_complete_files.append(self.sample_video.temp_sample_full_file_name)
+        else:
+            self._delete_sample_video(at_program_close=False)
+        self._sample_video = video
+
+    def get_main_window_display_name_and_icon(self) -> Tuple[str, QIcon]:
+        """
+        Generate the name to display at the top left of the main
+        window, indicating the source of the files.
+
+        :return: string to display and associated icon
+        """
+
+        if not len(self):
+            return _('Select Source'), QIcon(':/icons/computer.svg')
+        elif len(self) == 1:
+            # includes case where path is the only device
+            device = list(self.devices.values())[0]
+            return device.display_name, device.get_icon()
+        else:
+            non_pc_devices = [device for device in self.devices.values()
+                              if device.device_type != DeviceType.path]   # type: List[Device]
+            assert len(non_pc_devices) == len(self.volumes_and_cameras)
+            device_types = Counter(d.device_type for d in non_pc_devices)
+            if len(device_types) == 1:
+                device_type = list(device_types)[0]
+                device_type_text = self._map_plural_types[device_type]
+            else:
+                device_type = None
+                device_type_text = _('Devices')
+
+            if len(self.this_computer) == 1:
+                assert len(self.this_computer) < 2
+                assert len(self.this_computer) > 0
+
+                icon = QIcon(':/icons/computer.svg')
+                devices = list(self.volumes_and_cameras)
+                computer_display_name=self.devices[list(self.this_computer)[0]].display_name
+
+                if len(self.volumes_and_cameras) == 1:
+                    device_display_name = self.devices[devices[0]].display_name
+                else:
+                    assert len(self.volumes_and_cameras) > 1
+                    device_display_name = self._mixed_devices(device_type_text)
+
+                text = _('%(device1)s + %(device2)s') % {'device1': device_display_name,
+                                                                'device2': computer_display_name}
+                return text, icon
+            else:
+                assert len(self.this_computer) == 0
+
+                mtp_devices = [d for d in non_pc_devices if d.is_mtp_device]
+
+                if len(device_types) == 1:
+                    if len(self) == 2:
+                        devices = non_pc_devices
+                        text = _('%(device1)s + %(device2)s') % {'device1': devices[0].display_name,
+                                                                'device2': devices[1].display_name}
+                        if device_type == DeviceType.camera and len(mtp_devices) != 2:
+                            return text, QIcon(':/icons/camera.svg')
+                        return text, devices[0].get_icon()
+                    try:
+                        text_number = number(len(self.volumes_and_cameras)).number.capitalize()
+                    except KeyError:
+                        text_number = len(self.volumes_and_cameras)
+                    if device_type == DeviceType.camera:
+                        # Number of cameras e.g. 3 Cameras
+                        text = _('%(no_cameras)s Cameras') % {'no_cameras': text_number}
+                        if len(mtp_devices) == len(self.volumes_and_cameras):
+                            return text, non_pc_devices[0].get_icon()
+                        return text, QIcon(':/icons/camera.svg')
+                    elif device_type == DeviceType.volume:
+                        text = _('%(no_devices)s Devices') % dict(no_devices=text_number)
+                        return text, QIcon(':/icons/drive-removable-media.svg')
+                else:
+                    device_display_name = self._mixed_devices(device_type_text)
+                    icon = QIcon(':/icons/computer.svg')
+                    return device_display_name, icon
+
+
+# QStorageInfo, BackupLocationType
+BackupDevice = namedtuple('BackupDevice', 'mount, backup_type')
+
+# QStorageInfo, str, str, BackupLocationType
+BackupVolumeDetails = namedtuple('BackupVolumeDetails', 'mount name path backup_type '
+                                                        'os_stat_device')
+
+
+def nth(iterable, n, default=None):
+    "Returns the nth item or a default value"
+
+    return next(itertools.islice(iterable, n, None), default)
+
+
+class BackupDeviceCollection:
+    r"""
+    Track and manage devices (and manual paths) used for backing up.
+    Photos can be backed up to one location, and videos to another; or
+    they can be backed up to the same location.
+
+    If a BackupDevice's mount is None, then it is assumed to be
+    a manually specified path.
+
+    Backup devices are indexed by path, not id
+
+    >>> b = BackupDeviceCollection()
+    >>> len(b)
+    0
+    >>> p = BackupDevice(mount=None, backup_type=BackupLocationType.photos)
+    >>> p2 = BackupDevice(mount=None, backup_type=BackupLocationType.photos)
+    >>> v = BackupDevice(mount=None, backup_type=BackupLocationType.videos)
+    >>> pv = BackupDevice(mount=None,
+    ...                   backup_type=BackupLocationType.photos_and_videos)
+    >>> pv2 = BackupDevice(mount=None,
+    ...                   backup_type=BackupLocationType.photos_and_videos)
+    >>> b['/some/photo/path'] = p
+    >>> b
+    {'/some/photo/path':None <BackupLocationType.photos: 1> 0}
+    >>> b.device_id('/some/photo/path')
+    0
+    >>> b['/some/other/photo/path'] = p2
+    >>> del b['/some/other/photo/path']
+    >>> b['/some/video/path'] = v
+    >>> len(b)
+    2
+    >>> b.device_id('/some/video/path')
+    2
+    >>> b.device_id('/unknown/path')
+    >>>
+    >>> '/some/photo/path' in b
+    True
+    >>> b['/some/photo/path']
+    BackupDevice(mount=None, backup_type=<BackupLocationType.photos: 1>)
+    >>> len(b.photo_backup_devices)
+    1
+    >>> len(b.video_backup_devices)
+    1
+    >>> b['/some/photo/video/path'] = pv
+    >>> len(b.photo_backup_devices)
+    2
+    >>> len(b.video_backup_devices)
+    2
+    >>> del b['/some/photo/path']
+    >>> len(b.photo_backup_devices)
+    1
+    >>> len(b.video_backup_devices)
+    2
+    >>> b['/some/video/path'] = pv2
+    >>> len(b.photo_backup_devices)
+    2
+    >>> len(b.video_backup_devices)
+    2
+    >>> del b['/some/video/path']
+    >>> del b['/some/photo/video/path']
+    >>> len(b)
+    0
+    >>> len(b.photo_backup_devices)
+    0
+    >>> len(b.video_backup_devices)
+    0
+    """
+    def __init__(self, rapidApp=None):
+        self.rapidApp = rapidApp
+        self.devices = dict()  # type: Dict[str, BackupDevice]
+        # Set[path]
+        self.photo_backup_devices = set()  # type: Set[str]
+        self.video_backup_devices = set()  # type: Set[str]
+
+        self._device_ids = {}
+        self._device_id = 0
+
+    def __setitem__(self, path: str, device: BackupDevice):
+        if path in self.devices:
+            del self[path]
+        self.devices[path] = device
+        backup_type = device.backup_type
+        if backup_type in [BackupLocationType.photos,
+                           BackupLocationType.photos_and_videos]:
+            self.photo_backup_devices.add(path)
+        if backup_type in [BackupLocationType.videos,
+                           BackupLocationType.photos_and_videos]:
+            self.video_backup_devices.add(path)
+        self._device_ids[path] = self._device_id
+        self._device_id += 1
+
+
+    def __delitem__(self, path):
+        backup_type = self.devices[path].backup_type
+        if backup_type in (BackupLocationType.photos, BackupLocationType.photos_and_videos):
+            self.photo_backup_devices.remove(path)
+        if backup_type in (BackupLocationType.videos, BackupLocationType.photos_and_videos):
+            self.video_backup_devices.remove(path)
+        del self.devices[path]
+        del self._device_ids[path]
+
+    def __repr__(self):
+        s = '{'
+        for key, value in self.devices.items():
+            s += r'%r:%r %r %s, ' % (key, value.mount, value.backup_type,
+                                     self._device_ids[key])
+        s = s[:-2] + '}'
+        return s
+
+    def __contains__(self, key):
+        return key in self.devices
+
+    def __len__(self):
+        return len(self.devices)
+
+    def __getitem__(self, path):
+        return self.devices[path]
+
+    def __iter__(self):
+        return iter(self.devices)
+
+    def all_paths(self) -> List[str]:
+        return list(self.devices.keys())
+
+    def device_id(self, path: str) -> Optional[int]:
+        if path in self:
+            return self._device_ids[path]
+        return None
+
+    def name(self, path: str, shorten: bool=False) -> str:
+        """
+        :param path:
+        :param shorten: if True, and backup type is not an
+         automatically detected device, return the path basename
+        :return: device mount name, or path / path basename
+        """
+
+        if self.devices[path].mount is None:
+            if shorten:
+                return get_path_display_name(path)[0]
+            else:
+                return path
+        else:
+            mount = self.devices[path].mount  # type:  QStorageInfo
+            if not shorten:
+                return mount.displayName()
+            else:
+                name = mount.name()
+                if name:
+                    return name
+                else:
+                    return get_path_display_name(mount.rootPath())[0]
+
+    def backup_type(self, path) -> BackupLocationType:
+        return self.devices[path].backup_type
+
+    def multiple_backup_devices(self, file_type: FileType) -> bool:
+        """
+
+        :param file_type: whether the file is a photo or video
+        :return: True if more than one backup device is being used for
+        the file type
+        """
+        return ((file_type == FileType.photo and len(self.photo_backup_devices) > 1) or
+                (file_type == FileType.video and len(self.video_backup_devices) > 1))
+
+    def get_download_backup_device_overlap(self,
+               photo_download_folder: str,
+               video_download_folder: str) -> DefaultDict[int, Set[FileType]]:
+        """
+        Determine if the photo/video download locations and the backup locations
+        are going to the same partitions.
+
+        :param photo_download_folder: where photos are downloaded
+        :param video_download_folder: where videos are downloaded
+        :return: partitions that are downloaded and backed up to,
+         referred to by os.stat.st_dev
+        """
+
+        try:
+            photo_device = os.stat(photo_download_folder).st_dev
+        except FileNotFoundError:
+            photo_device = 0
+        try:
+            video_device = os.stat(video_download_folder).st_dev
+        except:
+            video_device = 0
+
+        downloading_to = defaultdict(set)  # type: DefaultDict[int, Set[FileType]]
+
+        if photo_device != video_device:
+            download_dests = (photo_device, video_device)
+        else:
+            download_dests = (photo_device, )
+
+        for path in self.devices:
+            try:
+                backup_device = os.stat(path).st_dev
+            except:
+                backup_device = 0
+            if backup_device != 0:
+                d = self.devices[path]
+                backup_type = d.backup_type
+                for download_device  in download_dests:
+                    if backup_device == download_device:
+                        if backup_type in (BackupLocationType.photos,
+                                           BackupLocationType.photos_and_videos):
+                            downloading_to[backup_device].add(FileType.photo)
+                        if backup_type in (BackupLocationType.videos,
+                                           BackupLocationType.photos_and_videos):
+                            downloading_to[backup_device].add(FileType.video)
+        return downloading_to
+
+    def get_manual_mounts(self) -> Optional[Tuple[BackupVolumeDetails, ...]]:
+        """
+        Get QStorageInfo, display name, and path for each backup
+        destination for manually specified backup destinations.
+
+        Display name is the path basename.
+
+        Lists photo backup destination before video backup destination.
+
+        Exceptions are not caught, however invalid destinations are accounted
+        for.
+
+        :return: Tuple of one or two Tuples containing QStorageInfo, display name,
+         and path. If no valid backup destinations are found, returns None.
+        """
+
+        assert len(self.devices)
+
+        paths = tuple(self.devices.keys())
+
+        if len(paths) == 1:
+            if not os.path.isdir(paths[0]):
+                return None
+            same_path = True
+            path = paths[0]
+            backup_type = BackupLocationType.photos_and_videos
+        else:
+            assert len(paths) == 2
+            photo_path = tuple(self.photo_backup_devices)[0]
+            video_path = tuple(self.video_backup_devices)[0]
+
+            photo_path_valid = os.path.isdir(photo_path)
+            video_path_valid = os.path.isdir(video_path)
+
+            if photo_path_valid and video_path_valid:
+                same_path = False
+            elif photo_path_valid:
+                same_path = True
+                path = photo_path
+                backup_type = BackupLocationType.photos
+            elif video_path_valid:
+                same_path = True
+                path = video_path
+                backup_type = BackupLocationType.videos
+            else:
+                return None
+
+        if same_path:
+            name = self.name(path, shorten=True)
+            mount = QStorageInfo(path)
+            os_stat_device = os.stat(path).st_dev
+            return (BackupVolumeDetails(mount, name, path, backup_type, os_stat_device), )
+        else:
+            photo_name = self.name(photo_path, shorten=True)
+            video_name = self.name(video_path, shorten=True)
+            photo_mount = QStorageInfo(photo_path)
+            photo_os_stat_device = os.stat(photo_path).st_dev
+
+            if same_device(photo_path, video_path):
+                # Translators: two folder names, separated by a plus sign
+                names = _('%s + %s') % (photo_name, video_name)
+                paths = '%s\n%s' % (photo_path, video_path)
+                return (BackupVolumeDetails(photo_mount, names, paths,
+                                            BackupLocationType.photos_and_videos,
+                                            photo_os_stat_device),)
+            else:
+                video_mount = QStorageInfo(video_path)
+                video_os_stat_device = os.stat(video_path).st_dev
+                return (BackupVolumeDetails(photo_mount, photo_name, photo_path,
+                                            BackupLocationType.photos,
+                                            photo_os_stat_device),
+                        BackupVolumeDetails(video_mount, video_name, video_path,
+                                            BackupLocationType.videos,
+                                            video_os_stat_device))
+
+    def get_backup_volume_details(self, path: str) -> BackupVolumeDetails:
+        """
+        For now only used in case of external mounts i.e. not auto-detected.
+
+        :param path: backup path
+        :return: named tuple of details of the backup volume
+        """
+
+        name = self.name(path, shorten=True)
+        device = self.devices[path]
+        if device.mount is not None:
+            mount = device.mount
+        else:
+            mount = QStorageInfo(path)
+        backup_type = device.backup_type
+        os_stat_device = os.stat(path).st_dev
+        return BackupVolumeDetails(mount, name, path, backup_type, os_stat_device)
+
+    def backup_possible(self, file_type: FileType) -> bool:
+        """
+
+        :param file_type: whether the file is a photo or video
+        :return: True if more a backup device is being used for
+        the file type
+        """
+        if file_type == FileType.photo:
+            return len(self.photo_backup_devices) > 0
+        elif file_type == FileType.video:
+            return len(self.video_backup_devices) > 0
+        else:
+            logging.critical("Unrecognized file type when determining if backup is possible")
+
+    def _add_identifier(self, path: Optional[str], file_type: FileType) -> Optional[str]:
+        if path is None:
+            return None
+        if file_type == FileType.photo:
+            return os.path.join(path, self.rapidApp.prefs.photo_backup_identifier)
+        else:
+            return os.path.join(path, self.rapidApp.prefs.video_backup_identifier)
+
+    def sample_device_paths(self) -> List[str]:
+        """
+        Return a sample of up to three paths on detected backup devices.
+
+        Includes the folder identifier (specified in the user prefs)
+        used to identify the backup drive.
+
+        Illustrates backup destinations for each of photo, video, such
+        that:
+        - If photos are being backed up to a device, show it.
+        - If videos are being backed up to a device, show it.
+        - If photos and videos are being backed up to the same device,
+          show that they are.
+
+        :return: sorted list of the paths
+        """
+
+        # Prioritize display of drives that are backing up only one type
+        both_types = self.photo_backup_devices & self.video_backup_devices
+        photo_only = self.photo_backup_devices - both_types
+        video_only = self.video_backup_devices - both_types
+
+        photo0 = nth(iter(photo_only), 0)
+        video0 = nth(iter(video_only), 0)
+        both0, both1 = tuple(itertools.chain(itertools.islice(both_types, 2),
+                                                           itertools.repeat(None, 2)))[:2]
+
+        # Add the identifier specified in the user's prefs
+        photo0id, photo1id, photo2id = (self._add_identifier(path, FileType.photo)
+                                        for path in (photo0, both0, both1))
+        video0id, video1id, video2id = (self._add_identifier(path, FileType.video)
+                                        for path in (video0, both0, both1))
+
+        paths = [path for path in (photo0id, video0id, photo1id, video1id, photo2id, video2id)
+                 if path is not None][:3]
+
+        if len(paths) < 3:
+
+            unused_photo = self.photo_backup_devices - {path for path in (photo0, both0, both1)
+                                                        if path is not None}
+            unused_video =  self.video_backup_devices - {path for path in (video0, both0, both1)
+                                                         if path is not None}
+            photo1, photo2 = tuple(itertools.chain(itertools.islice(unused_photo, 2),
+                                                               itertools.repeat(None, 2)))[:2]
+            video1, video2 =  tuple(itertools.chain(itertools.islice(unused_video, 2),
+                                                               itertools.repeat(None, 2)))[:2]
+            photo3id, photo4id = (self._add_identifier(path, FileType.photo)
+                                        for path in (photo1, photo2))
+            video3id, video4id = (self._add_identifier(path, FileType.video)
+                                        for path in (video1, video2))
+
+            paths += [path for path in (photo3id, video3id, photo4id, video4id)
+                      if path is not None][:3 - len(paths)]
+
+        return sorted(paths)
+
+    def backup_destinations_missing(self,
+                                downloading: DownloadingFileTypes) -> Optional[BackupFailureType]:
+        """
+        Checks if there are backup destinations matching the files
+        going to be downloaded
+        :param downloading: the types of file that will be downloaded
+        :return: None if no problems, or BackupFailureType
+        """
+        prefs = self.rapidApp.prefs
+        if prefs.backup_files:
+            photos = downloading in (DownloadingFileTypes.photos,
+                                     DownloadingFileTypes.photos_and_videos)
+            videos = downloading in (DownloadingFileTypes.videos,
+                                     DownloadingFileTypes.photos_and_videos)
+
+            if prefs.backup_device_autodetection:
+                photo_backup_problem = photos and not self.backup_possible(FileType.photo)
+                video_backup_problem = videos and not self.backup_possible(FileType.video)
+            else:
+                photo_backup_problem = photos and not validate_download_folder(
+                    path=prefs.backup_photo_location,
+                    write_on_waccesss_failure=True
+                ).valid
+                video_backup_problem = videos and not validate_download_folder(
+                    path=prefs.backup_video_location,
+                    write_on_waccesss_failure=True
+                ).valid
+
+            if photo_backup_problem:
+                if video_backup_problem:
+                    return BackupFailureType.photos_and_videos
+                else:
+                    return BackupFailureType.photos
+            elif video_backup_problem:
+                return BackupFailureType.videos
+            else:
+                return None
+        return None
+
+
+class FSMetadataErrors:
+    """
+    When downloading and backing up, filesystem metadata needs to be copied.
+    Sometimes it's not possible. Track which devices (computer devices,
+    according to the OS, that is, not the same as above) have problems.
+    """
+
+    def __init__(self) -> None:
+        # A 'device' in this class is the st_dev value returned by os.stat
+        self.devices = set()  # type: Set[int]
+        self.archived_devices = set()  # type: Set[int]
+        # device: FsMetadataWriteProblem
+        self.metadata_errors = dict()  # type: Dict[int, FsMetadataWriteProblem]
+        # scan_id / device_id: Set[device]
+        self.worker_id_devices = defaultdict(set)  # type: DefaultDict[int, Set[int]]
+
+    def add_problem(self, worker_id: int,
+                    path: str,
+                    mdata_exceptions: Tuple[Exception]) -> None:
+
+        dev = os.stat(path).st_dev
+
+        if dev not in self.devices:
+            self.devices.add(dev)
+
+            name, uri, root_path, fstype = fs_device_details(path)
+
+            problem = FsMetadataWriteProblem(
+                name=name, uri=uri, mdata_exceptions=mdata_exceptions
+            )
+
+            self.metadata_errors[dev] = problem
+
+            if worker_id is not None:
+                self.worker_id_devices[worker_id].add(dev)
+
+    def problems(self, worker_id: int) -> List[FsMetadataWriteProblem]:
+        problems = []
+        for dev in self.worker_id_devices[worker_id]:
+            if dev not in self.archived_devices:
+                problems.append(self.metadata_errors[dev])
+                self.archived_devices.add(dev)
+        return problems