Yes, it is FAT. I'd suggest you plug your n810 into a PC and then run chkdsk on the drives. I've had to do this a few times: /media/mmc2 does seem to get corrupted rather easily.

If you have a swapfile on this drive then you might want to turn it off first: I'm not sure but I suspect mounting the drive on a PC while the swapfile is enabled may be part of the problem.