There's no known issues with the 922 deleting things on it's own, especially 300GB worth (we normally see all gone, or none gone). If it's the first time it happened, sorry, but I'd ask for you to wait (backup some stuff too). If it happened again, then I would replace the receiver.
Oh, and hard reset as always
For the people who had this problem, if you send me account info (including software version on receiver), I'll report the problems so that any trends can appear