“Failed to eject media; one or more volumes on the media are busy.”
That is an error message you may come across from time to time, and of course the obvious thing to do is make sure no programs are still trying to access a CD or DVD in your disc drive. And this includes programs that aren’t actively using the device, but are nonetheless keeping it “busy”, like if you have a media player open with files or titles from the disc still in its playlist. But once you have ruled all that out, you still might be left with the inability to eject the disc and get on with things, so hopefully some information here will be of use to you.
In most cases, the following command run in a terminal or via Alt+F2 will successfully unmount the disc, and then eject the tray:
sudo umount -l /media/cdrom0 && eject
Please note: since Ubuntu now automagically creates mount points for discs based on the label, /media/cdrom0 may not work for you, so you may need to ascertain the correct path for the current disc with this command: