Thanks again for the advice and support. I've been running the 160GB from an old 8300HD (cable company now defunct) with no trouble (a little pixelation but it's still very infrequent). I've since found my SD Tivo with the upgraded 500GB and pulled the drive (needs both T10 and T15 drivers, but not security).
I added a post to the sticky thread (which itself has been incredibly valuable, I think I've read it all!). But to close things off here, this drive is a 500GB Seagate DB35.3 model ST3500830ACE. I bought it on eBay about 3-4 years ago and upgraded my 80GB SD TiVo. It was sold specifically to be used with a TiVo and in fact came preformatted and with whatever special stuff TiVo uses already loaded. It worked well for about 2 years (2010-2012) both on Comcast (FL) and up here, until the cable company folded and we got dumped onto Bell Fibe. Meanwhile Comcast went the Rogers route and cut back analogue to 2-26 or whatever (probably now requires a DTA), so we invested in a CableCard and HD TiVo for Comcast.
I don't recall any noise issues with the drive. Can I just drop this into my 8300HD and then force a reformat? This is different from the 160GB, which did require a reformat to clear the old recordings but appeared to be able to record new shows as-is. My HD TiVo only has 320GB but I can pull recordings off it onto an external connected to my notebook, so its capacity is essentially unlimited (just a lot of work when there's a lot recording).