I'm going through the same basic scenario. the 3 TB fantom drive that I've had connected for a couple years stopped being recognized. After a couple reboots, a screen came up showing it was scanning the external drive for errors, said it would take several hours. let it run, after a day it said it found and fixed about 46,000 errors, but was only 7.10% complete. Tried a new drive, but it doesn't seem to want to even recognize one is plugged in. I'm thinking it has something to do with the DVR eSata port.
I already got a Genie with a 4 TB drive hooked up 2 months ago, so I guess I'll just live with the reduced capacity with my HR24-500