diff --git a/src/calibre/devices/usbms/device.py b/src/calibre/devices/usbms/device.py
index 6f8e62eea9..613161cd5f 100644
--- a/src/calibre/devices/usbms/device.py
+++ b/src/calibre/devices/usbms/device.py
@@ -407,10 +407,10 @@ class Device(DeviceConfig, DevicePlugin):
             print
             from pprint import pprint
             pprint({'bsd_drives': bsd_drives, 'mount_map': mount_map, 'drives': drives})
-        if drives.get('main') is None and drives.get('carda'):
-            drives['main'] = drives.pop('carda')
         if drives.get('carda') is None and drives.get('cardb'):
             drives['carda'] = drives.pop('cardb')
+        if drives.get('main') is None and drives.get('carda'):
+            drives['main'] = drives.pop('carda')
         if drives.get('main') is None:
             raise DeviceError(_('Unable to detect the %s mount point. Try rebooting.')%self.__class__.__name__)
         pat = self.OSX_MAIN_MEM_VOL_PAT