i've read about this happening before with flash drives. if it was bought online it may be the same problem, people are using small amounts of memory yet programming it to show more than that to sell them on sites like ebay and such. if this happened theres not anything you can do about it except try and get your money back, which is unlikely to happen.
heres a link to a similar thing.
Untitled Document
they allow a download program to check if it is a dodgy drive.