Simple fix (Windows 7 Professional 64-bit):
Change the drive letter, and if you want to keep the same letter, change it back.
The drive can now be unmounted.
When you change the drive letter, it disconnects the drive from all processes that might be using it, as the warning message states. Once that happens, you can then unmount it the usual way.
If you don't know how to change drive letter, follow these instructions.
Control Panel --> Administrative Tools --> Computer Management -->
Storage --> Disk Management (wait ~10 seconds for information to appear.)
Select the problem drive under volume heading.
Scroll to that drive in the lower part of window.
Right-click on the drive, and select Change Drive Letter and Paths.
Change the drive letter to one not reserved for another drive.
Acknowledge the warning message.
If you wish the drive to retain the original letter, simply switch it back. The drive can now be unmounted.
5Still a daily problem on Windows 10 Pro (1703, 15063.413, x64). – Tamás Bolvári – 2017-09-16T23:56:34.980
1Windows 10 1809, still the same.... Why don't they just write WHICH program is using it.... – Yvon – 2019-03-27T23:33:10.617
Related: Can Windows tell me what is using my USB drive?
– krubo – 2019-05-31T12:35:42.6675Sleep the box, remove the drive once the box is asleep. – Daniel R Hicks – 2014-04-11T11:44:21.643