Is a bit difficult to explain. Let me try
"Switch to Sdcard" requires and sdcard formated as an external storage. Most roms, when you insert an sdcard, ask you if you want to use it as portable storage or as your device main storage.
Also, since there's no API to retrieve the path of the external sdcard directly, the program uses a
common trick of figuring the path based on app external storage path. This should work on most devices, but it might be the case where this doesn't happen as some e-ink manufacturers are not good implementing compatible devices.
AFAIK Likebook devices are pretty OK with android compatibility, so please check that you didn't format the sdcard as internal storage.