From few days, I've the following popup appearing on my screen floating on the top with all the buttons grayed out and no possibility to close it down no matter what I do. Even it's appearing on full-screen applications. It says:

The disk "WD_1TB" wasn't ejected because one or more programs may be using it.
To eject the disk immediately, click the Force Eject button.
'Cancel' 'Force Eject...'

It's quite annoying and it happens from time to time when I'm trying to force eject my HDD and it's failing.

In Linux, I could easily run xkill, but it doesn't work on OSX.

I've tried  -> 'Force Quit...', but it's not appearing on the list and restarting Finder doesn't help.

Any suggestions how to get rid of it without being force to reboot?

With ZFS-OSX, sometimes when I try to force eject my HDD and it's failing, the following popup appears on my screen – floating on the top with all the buttons grayed out – and no possibility to close it no matter what I do:

The disk "WD_1TB" wasn't ejected because one or more programs may be using it.

To eject the disk immediately, click the Force Eject button.

Cancel    Force Eject…

It's quite annoying. It even appears with full-screen applications.

In Linux, I could easily run xkill, but it doesn't work on OS X.

I've tried  -> 'Force Quit…', but it's not appearing on the list and restarting Finder doesn't help.

Any suggestions how to get rid of it without being forced to reboot?