I don't know the answer to that?
However I suspect (in my case anyway) because the drive was formatted in Windows as a DOS partition the problems emanate from UNIX reading the blocks from zero instead of the offset created when windows formatted the drive.
see my other post for a brief description about FAT drives created in windows vs. UNIX.
I have another USB formatted in ext2 to test that theory but have not had the time yet.
If you find the answer please share your results.

Peace & Prosperity,
JC