I was in the middle of playing Assassin's Creed 2 when a message pops up in game telling me the storage device has been removed. I'm using the, I believe it's 4 GB xbox, with no hard drive. When I exited the game to check on the memory unit, I realized it didn't have any save data for AC2. I still had my achievements, but no save files. It was working perfectly fine last night so I'm not entirely sure what happened. Does anyone know what causes this?