[SCION]

Why can't you continue to use calibre? Send some books to internal memory, send others to the card. Calibre gives you a choice of where to send books. Perhaps I'm overlooking something that should be obvious.

I know it's not ideal, but at this stage it's either return the device (if that's an option) or come up with workarounds.

Edit: Or you can break up your library into two. One library for internal storage and another library for the card. It's easy to switch between libraries within calibre while the device is connected.