Windows really should tell you what program is using the drive, but it doesn't. You're going to have to do some detective work.
Start by doing what Windows recommends: Wait a bit, then try again.
But keep busy while you wait. Make sure you don't have the drive open
in Windows Explorer; in fact, you may want to close all of your Windows
Explorer windows, just in case. Next, examine the system tray
to make sure that a program...