Can't eject because one or more...using it ?

Yes, it is very annoying.

You can try to identify the app or process accessing the volume, by using the following command in the Terminal app:

lsof  <path-to-mounted-volume>

You need to replace "<path-to-mounted-volume>" with the correct information. The easiest way is by first typing "lsof " with at least one space after it. Then drag & drop the volume from the Finder onto the open Terminal window which will auto fill the correct path. To see the mounted volumes in the Finder, view the "/Volumes" folder.

James at 118 wrote:

The Finder knows every nanosecond exactly what applications are using what. Why do I get this clueless, vaguely ominous warning when I want to eject a drive?

Because the process that is preventing the ejection is likely some background task. You would need fairly advanced UNIX command-line skills in the Terminal to track down the process and kill it.

HWTech wrote:

You can try to identify the app or process accessing the volume

It will tell you if you ask. Run:

diskutil unmount <path-to-mounted-volume>

If it fails, it will tell you which process prevented the unmount. You can then quit that process.

Thanks.