Bumping this topic because I just encountered the error #51 in a slightly different fashion.
I have 2 HD-Tivos. One flaked out over the weekend, apparently due to an access card problem. Unfortunately that's the one that's connected to my main HT, and has better OTA reception - so I really need something there to record OTA shows that I can't get via satellite in HD, such as Reaper on CW. So I figured I'd try swapping hard drives with the working Tivo, though I thought there was a chance that wouldn't work - and that's exactly what I found. So that part is no surprise.
BUT... when I replaced the drive in the working Tivo (which is itself a bigger drive recently copied from the original, but has been working fine), I still got error #51. Since it's a copy on a secondary unit I'm doing a clear & delete (in progress), but I'm a little concerned now that the original drive, if I ever put it back, would have problems. (Among other things, that drive has a bunch of recordings that I want to preserve and I haven't delved into "taboo territory" with respect to that.)
Any thoughts on why the re-swap still caused an error #51, and whether that would also be the case with the original drive?