Progress!

Booting without the external card in, I can now see the 'internal' one, however, I get an error from it at boot saying "Memory card incorrectly formatted or corrupted.

Using the file browser, I can repair or format the drive, but neither fixes the problem. I've used MKFS thoiugh penguinbaits GUI to reformat the disk (using a couplle different formats to see if there's a difference) although they all say successful, the filebrowser still says the card is incorrectly formatted or corrupted.

I used fdisk to delete the partition on the disk, and to create a new one, but that was also unsuccessful.