The problem with these little cards is that they are basically ungrounded hard drives. They are the only USB-drive things I still safely eject.

If your DS can read it, it could be that only the last file, the one it was transferring, is corrupted. Your DS would just ignore it, like other files you can dump on your M3.

How can't your computer read the card anymore? Does it lock up? Does it just not see the card anymore?