Thanks Mac, that article explains why a run of CHKDSK fixes the issue and things then go bad again over time.
If it is Windows, changing the drive may not fix the issue. I add that I never encountered this with XP over many years of video editing, admittedly with drives being replaced every couple of years by bigger ones.
Running CHKDSK /R again now to fix more problems.
I'll shortly pull the drive out and get the SD details (and try to find the receipt!
).