Post some screenshots of the Diagnostics screen for the hard drive. Or at least write down the numbers that appear for total disk space, remaining disk space, and used disk space.
To access the Diagnostics screen, do the following:
1. Put the cable box into standby (either by hitting the Power button on your remote or the one on the front of the cable box).
2. Within one second of putting the cable box into Standby, press the "OK/Select" button (either on your remote or on the front of the cable box).
3. A white "Diagnostics" screen will appear within a few seconds. Use the arrow keys to go to the Hard Drive page (I think it is named d13). and then press the "OK/Select" button.
4. You'll see stats about your hard drive showing you the remaining space, total space, and used space.
If you really want more hard drive space, you can swap out the hard drive with a 1 TB drive by following the directions in this
post from this
(Of course your cable company doesn't want you opening the box, so you need to take care and buy a replacement security tab and get the special screwdriver bit to remove the three special screws from the back of it, but the whole upgrade is really quite easy).