Yes, thanks… chkdsk /f showed no errors.
I also did chkdsk /r, which also showed no errors for clusters with ‘user files’. Once it got to empty clusters it was taking forever (as in 20 hours and still lots to go) so I aborted before it was done.

Thus it doesn’t seem like it is an HDD issue, but I can’t really be 100% sure whether the slow /r progress was just because of a 1TB drive versus it was getting lots of errored (but empty) clusters.

(Hence why I’ll replace the HDD if I eventually punt and do a full re-install from scratch)