The "format in a PC" idea is good but in the end, cards' file systems crapping out is just a fact of life with memory cards. I've never had a memory card for any length of time that didn't give file system problems at one point or another. That's why its important to back up your storage card as well as your Pocket PC. Usually, after I do a full backup of my PPC to a storage card with Sprite Backup, I'll copy the entire contents of the memory card to my PC via a card reader.