Don't delete anything from the card until you have checked with warranty support for the camera and the memory stick. Laws are different where I am, but here you would be able to get your money back on the grounds of defective goods. You've done right in recovering your data. It very possibly could be a fault in the camera, and the data is evidence of what the fault may be. Could be the firmware, and just needs the latest version. But you paid for something that works, and it doesn't.