diff --git a/src/calibre/devices/usbms/device.py b/src/calibre/devices/usbms/device.py
index dcae8e3942..77d7795f6b 100644
--- a/src/calibre/devices/usbms/device.py
+++ b/src/calibre/devices/usbms/device.py
@@ -411,6 +411,8 @@ class Device(DeviceConfig, DevicePlugin):
             drives['carda'] = drives.pop('cardb')
         if drives.get('main') is None and drives.get('carda') is not None:
             drives['main'] = drives.pop('carda')
+        if drives.get('carda') is None and drives.get('cardb') is not None:
+            drives['carda'] = drives.pop('cardb')
         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