This happens to me when show to my wife how to move a cartoon film for our son to an USB drive.
As you can see at the attached video (don't pay attention to the second 11 glitch, I moved a window by mistake while recording the video), after the progress bar has arribed to the end, we must wait for a minute (or more) before can unplug the USB disk. If don't pay attention to this notification, you cannot unplug the disk safely.
Maybe can be included into 15-minute bug initiative?