I don't think it's my imagination, but after using mp3tag to edit tags on files located on a USB-connected HDD (Western Digital "My Passport" in this example), asking Windows to eject the drive results in a message stating the the drive is still in use, even though all apps (including mp3tag) have been closed. This occurs in Windows 7, 64-bit Pro and, I believe, in the Windows 10 version as well.
Download Sysinternals Process Explorer, start it, go to the pulldownmenu "Find" -> "Find Handle or DLL" and type in the name of a blocked mp3.
If the mp3 is blocked by any application, process explorer will show it.
The OS hinders an external storage to be removed if a single file is still open, when a file handle exists.
The OS then blocks this file for other access.
If a file is blocked, then the whole drive cannot be ejected.
You can find with the help of the process explorer the file that is still blocked.