I found that problem too with some usb pendrives, especially on playback of recorded HD channels. I now use Freecom portable hard drive which works ok most of the time.Reciever seems much more stable when connected with a faster hard drive.
You can go into the receiver's USB menu and check for your device's speed. I found that pendrives I tested were registering as slow compared to my Freecom, which I assume is why it's more stable.