I’m transferring some apps downloaded for my Atari ST (don’t ask) to floppies that are formatted with 80 tracks and 8 or 9 sectors, on double density disks (formatted on the ST). From what I understand these are MS-DOS readable but not exactly FAT format. Anyway, they don’t seem to mount by default on Ubuntu as it doesn’t know what format they are, but forcing a mount with this seems to work:
sudo udisks --mount /dev/fd0
seems to do the job (tip from here)