For fun, I checked my external drive while booted in Windows. Oh no! It needed to be defragmented! Defrag recommended CHKDSK as well.
Big mistake: One or both of them scattered a lot of my backup files (including the backups of this blog - quite a few entries gone until they turn up again). I'm afraid to look at my music...or my collection of Futurama episodes. Only fix is to hunt through the 'found' folder and put stuff away manually. Big mess.