I'm quite confident that you have got an (bit) error in the File Allocation Table (FAT).
This happens when the final pointervalue deviates from 'End of Table' (null) usually by a single bit.
I've seen this error symptom previously on PC systems where I had access to adequate tools (Norton) to perform the recovery.
An similar error was reported earlier this week by Jungan and akheron:
http://www.aximsite.com/boards/showthread.php?t=125229
Unfortunately no fix on the Ax is known - to me, that is .... Sorry !