Some of the HDDs were rescued from binned cable PVRs, all were Seagate - so
it was no surprise when a few started clicking and tangling up FAT chains.

Since taking out the old PC and firing up a spare, most of the drives are
behaving now, a couple started with the blue screen of CHKDSK during boot
but seemed fine after that. One failed to detect and still does on the new
PC, another prevented boot but was recoverable on the new PC.

Any tips on narrowing down the exact cause?

PSU ripple could be it - but I only have a basic load tester.

Thanks for any help.

PS: Drives were also getting jangled on a JMJ raid card I was using
unconfigured to get more USB sockets.