I'm going to use a couple of made up numbers for the purpose of illustration.
You had a Tivo that you sold. It had a Tivo Service Number of 123 (that's one of those made up numbers, the real one would be much longer and start with the first part of the model number).
Before you sold it you made an image file, by which I assume you mean truncated backup.
You restored that image onto the refurb they sent you and tried to boot in the TiVo you still have which has a TSN of 456.
The hard drive was expecting to boot up in the machine with the 123 TSN, and the TiVo was expecting a drive which expected to be booted in a machine with the TSN of 456. When you get that kind of conflict between hard drive and motherboard the TiVo throws an error 51 message, and then proceeeds to re-write stuff on the drive to make it confirm to the TSN of the motherboard. Once it finishes doing that, it should work fine in that TiVo, but any recordings made if and when the drive was in the 123 machine are no longer accessible, because they're tied to the old TSN (although in your case if the image from which you restored was a truncated one--that's how people usually do it--, there wouldn't be any recordings yet).
As for the rest of your post, I fear I cannot offer any help, but you might ask about that particular model drive on the Drive Upgrade and Expansion thread
http://www.tivocommunity.com/tivo-vb/showthread.php?t=370784
to see if anybody there knows anything about it.