diff --git a/src/calibre/devices/usbms/driver.py b/src/calibre/devices/usbms/driver.py
index 700a072c5b..f871bc20e7 100644
--- a/src/calibre/devices/usbms/driver.py
+++ b/src/calibre/devices/usbms/driver.py
@@ -85,7 +85,7 @@ class USBMS(CLI, Device):
         elif on_card == 'cardb' and not self._card_b_prefix:
             raise ValueError(_('The reader has no storage card in this slot.'))
         elif on_card and on_card not in ('carda', 'cardb'):
-            raise DeviceError(_('The reader has no storage card in this slot.'))
+            raise DeviceError(_('Selected slot: %s is not supported.' % on_card))
 
         if on_card == 'carda':
             path = os.path.join(self._card_a_prefix, self.EBOOK_DIR_CARD_A)
diff --git a/src/calibre/gui2/main.py b/src/calibre/gui2/main.py
index f7c00fc0c4..0a77839ce1 100644
--- a/src/calibre/gui2/main.py
+++ b/src/calibre/gui2/main.py
@@ -785,7 +785,7 @@ class Main(MainWindow, Ui_MainWindow, DeviceGUI):
 
     def _add_books(self, paths, to_device, on_card=None):
         if on_card is None:
-            on_card = self.stack.currentIndex() >= 2
+            on_card = 'carda' if self.stack.currentIndex() == 2 else 'cardb' if self.stack.currentIndex() == 3 else None
         if not paths:
             return
         from calibre.gui2.add import Adder
@@ -795,7 +795,7 @@ class Main(MainWindow, Ui_MainWindow, DeviceGUI):
                 Dispatcher(self.__adder_func))
         self._adder.add(paths)
 
-    def _files_added(self, paths=[], names=[], infos=[], on_card=False):
+    def _files_added(self, paths=[], names=[], infos=[], on_card=None):
         if paths:
             self.upload_books(paths,
                                 list(map(sanitize_file_name, names)),