After the full format and running Malwarebytes, suddenly Avast (been installed for some time) found a rootkit thingee. Maybe MWB shamed it into action...
Then tried to put all my video back no the drive and my sync program couldn't get going. I clicked on the drive in Windows Explorer and the green loading line went slowly across from left to right.
Drive removed from machine and replaced with new WD Black, which works well.
Removed/dead drive was is Seagate NAS drive. More tests on it on my external SATA dock before it goes back to the shop.