Yeah, looking around t'interweb, it seems to be a "thing". Thing is, this drive has now passed a ten hour checkdsk scan and an almost as long SMART scan (HDDScan), and it works perfectly in two different SATA slots on a different, newer, PC. As far as I know, it's only in the old PC with it's 11 year old mobo that it can't be seen. But that said, 4 other drives and a DVD drive work fine, in any slot. So there's obviously something about this particular HDD that stops it shaking hands. And it's something that happened over the weekend.

I've stuck on the latest BIOS and chipset drivers, but without any improvement. It's working OK through USB, and there's a card in a PCIE slot giving USB3 with a spare socket, so I'll mount it inside but connect it via USB.

Yeah, that is SOOO annoying and a real issue with Windows: the way one misbehaving process can hang or crash the entire OS, and there's no way to kill it.