I went through similar problems on 3 different 8000HD boxes and "fixed" it the last time (before calling TWC to come out and replace another one!) by doing the following, YMMV:
1. Disconnect power from the unit for 30 or more seconds.
2. While holding down the power button, reconnect power and continue to hold down the power button until the clock displays the correct time. (You'll hear "lots" of HD activity.)
3. Turn the unit on, and after the OS loads, press "List" to see if the HD error went away.
Again, this is easy to do to see if it "clears up" your issue. If not, I'm betting a replacement will be required.