Well the issue is that there are so many variations of "FAT" and some just don't play nice with MorphOS (and sometimes even the HW causes issues).

I don't know the details, but I do remember that wiping a drive and setting it up as FAT???? under OSX tends to not work with MorphOS.

So if you setup the drive in OSX it might help if you post a screenshot to let us see the details. You could also set it up as HFS (not HFS+) or just use another drive.