How to kill the popup in OS X when a disk can't be ejected

Question

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?

Answer 1

The process is UnmountAssistantAgent, therefore can be killed using Activity Monitor, or by running:

killall UnmountAssistantAgent