>One W7 laptop with normal HD sometimes becomes unresponsive, showing HD performance ~10MB, which is not really fun.
>Rebooting fixes it - guesstimate is that disc interface downscales from SATA to PIO (which I saw a desktop machine do reguöarly under XP, but there the "new" classification stayed over reboot and had to be corrected manually)...

I've seen 2 cases with modern laptops where the disk performance dropped to about 1% of normal. There were no SMART errors, no corresponding errors in the System event log and in both cases the drives passed the "thorough" manufacturer's diagnostics. In both cases I had to replace the drives, which restored normal system performance. Luckily in both cases it was still possible to image the drives and then restore to new hardware.

So, these current-generation 500+ GB 2.5" magnetic drives can suffer some sort of soft failure which greatly reduces their performance but doesn't make them actually "fail" per se.

I don't know if this is relevant to your case - in the cases I saw, once the performance dropped it never came back, even after cold rebooting. Still, I thought I'd mention it in case what you're seeing is some intermittent or "early warning" case of the same thing.

Replacement HDs are cheap these days (even SSDs are reasonable), it might be worth replacing that drive while it's still "functional" and you can successfully clone it.