As detailed at http://ocaoimh.ie/how-to-umount-when-the-device-is-busy/ if you have an external drive that you’ve right-clicked on your desktop and clicked ‘Unmount’ or ‘Eject’ but it says “An application is preventing the volume ‘(name)’ from being unmounted.”, there is a way to figure it out.

df

This will list your drives to see what the device name is. Now:

fuser -m /dev/devicename

Now you know what you need to fiddle with to get it to eject!