It's a bad memory
I am not impressed with SanDisk memory. Again. I should know better.
Just bought a Canon ELPH SD200 yesterday (a slip-into-the-pocket tourist camera), and since the store had SanDisk SD cards on sale, picked up half a gig.
Worked fine in the camera, then when I slipped it into the card reader, my desktop machine reported "corrupt" and unreadable. The same with my laptop. Every round trip back into the camera required a card format.
The store replaced the card, and yup...happened again.
After getting my money back, I picked up Lexar High-speed SD memory. Works fine. I've always sworn that the least I'd use was Lexar memory; I have no idea why I succumbed this time. I guess I wasn't thinking, just because it was this new-fangled SD stuff...
*sound of head banging wall*
All the best, James