Nicer error message when the user has set the default send to device action incorrectly

src/calibre/devices/errors.py

@@ -104,3 +104,9 @@ class ControlError(ProtocolError):
         if self.desc:
             return self.desc
         return "Unknown control error occurred"
+
+class WrongDestinationError(PathError):
+    ''' The user chose the wrong destination to send books to, for example by
+    trying to send books to a non existant storage card.'''
+    pass
+

src/calibre/devices/usbms/device.py

@@ -15,7 +15,8 @@ import os, subprocess, time, re, sys, glob
 from itertools import repeat
 
 from calibre.devices.interface import DevicePlugin
-from calibre.devices.errors import DeviceError, FreeSpaceError
+from calibre.devices.errors import (DeviceError, FreeSpaceError,
+        WrongDestinationError)
 from calibre.devices.usbms.deviceconfig import DeviceConfig
 from calibre.constants import iswindows, islinux, isosx, isfreebsd, plugins
 from calibre.utils.filenames import ascii_filename as sanitize, shorten_components_to
@@ -976,9 +977,17 @@ class Device(DeviceConfig, DevicePlugin):
 
     def _sanity_check(self, on_card, files):
         if on_card == 'carda' and not self._card_a_prefix:
-            raise ValueError(_('The reader has no storage card in this slot.'))
+            raise WrongDestinationError(_(
+                'The reader has no storage card %s. You may have changed '
+                'the default send to device action. Right click on the send '
+                'to device button and reset the default action to be '
+                '"Send to main memory".')%'A')
         elif on_card == 'cardb' and not self._card_b_prefix:
-            raise ValueError(_('The reader has no storage card in this slot.'))
+            raise WrongDestinationError(_(
+                'The reader has no storage card %s. You may have changed '
+                'the default send to device action. Right click on the send '
+                'to device button and reset the default action to be '
+                '"Send to main memory".')%'B')
         elif on_card and on_card not in ('carda', 'cardb'):
             raise DeviceError(_('Selected slot: %s is not supported.') % on_card)
 

src/calibre/gui2/device.py

@@ -23,7 +23,7 @@ from calibre.gui2 import (config, error_dialog, Dispatcher, dynamic,
 from calibre.ebooks.metadata import authors_to_string
 from calibre import preferred_encoding, prints, force_unicode, as_unicode
 from calibre.utils.filenames import ascii_filename
-from calibre.devices.errors import FreeSpaceError
+from calibre.devices.errors import FreeSpaceError, WrongDestinationError
 from calibre.devices.apple.driver import ITUNES_ASYNC
 from calibre.devices.folder_device.driver import FOLDER_DEVICE
 from calibre.devices.bambook.driver import BAMBOOK, BAMBOOKWifi
@@ -1450,6 +1450,9 @@ class DeviceMixin(object): # {{{
                                  'is no more free space available ')+where+
                                  '</p>\n<ul>%s</ul>'%(titles,))
                 d.exec_()
+            elif isinstance(job.exception, WrongDestinationError):
+                error_dialog(self, _('Incorrect destination'),
+                        unicode(job.exception), show=True)
             else:
                 self.device_job_exception(job)
             return