Hi Mitchell, thanks for the help. Actually it seems you are right. I see now my problem is more specific.
It happens when I manually copy/paste from my MediaMonkey song library into the new folder (something I actually do a lot). Once the progress box is appears, you can't do anything with the destination folder (including minimize, close or paste more folders in there) until it is finished . I guess this could be a MM-specific problem, but I know it used to work with XP as I've been doing it for years.
PS: I tested just copying some movie folders to USB as your suggestion. It worked as you said, but when I hit "Cancel" (once I could see it was working) the "Cancel" progress box sits there for many minutes until I pulled the USB out. Another problem I guess...