Hey, thanks for the fast response :-) It's a shame you couldn't get it fixed. Such a bizarre issue to spontaneously occur, eh.
I can't find a little switch anywhere, so I guess I may have to admit defeat. Fortunately it's only a 32GB pen drive so not a huge sacrifice, although it had been a trusty tool until this issue.
Csound1, all of the options are grayed out - both 'Erase' and 'Erase Free Space'. On all of the tabs the only option that is available is 'Verify Disk' and even that produces no further options. The current format is MS-DOS (FAT32).