I bought the device yesterday and had a similar problem. After using the device about 5 hours, suddenly it reboots and lost all the data. Reports only that "internal memory card format is not supported". In settings -> memory it says that the internal memory card is corrupted.

Wen't to Nokia Flagship store today and in 10 minutes time I got a new device. I'm just hoping that this doesn't happen again. Luclily, my wife's device (bought also yesterday at the same time) works like a charm.