@hfpop: AFAIK, the (annoying) way to have it refresh the thumbnail is to delete it *from the UI* (that'll remind you of something from the Kindle days
), then restore it *over USBMS*. Trying to be sneaky will just lead to Nickel shrugging at you and keeping on with its cached thumbnail
.
Note that if it's currently assigned to a KFMon watch, that'll probably cause KFMon to whine about a watched file being gone at some point (either straight away, or after the USBMS unplug), which will trip a safety feature (meaning it'll abort), so you'll then have to reboot on top of that...