Solved it! I had to do the one thing they say you should NOT do which is to unplug the SATA drive power while the system is running. When I did so I got a warning message saying basically "Don't do that". I turned off the DVR, waited 15 minutes, plugged the power plug back into the SATA drive.
I turned on the DVR and got a message saying "This drive needs to be formatted for this DVR and will take approximately 2 minutes". Fantastic! I waited and after about 30 seconds it said it was done. I pressed "LIST" on the remote and now total space is 13%, and thankfully, all of my other recordings are still there... whew!