Mac Mini can't unmount disk in Recovery Mode
Way by Terminal..

Use diskutil list in a Terminal instance to list all volumes and find yours.

Then do hdiutil eject disk?

Replace the ? with your disk number, making sure you are not trying to eject the main hard drive :) It is probably called "Disk1"

Here is a Sample when I insert a USB stick and run diskutil list



disk0 is my hard drive and I can NOT eject that one

disk1 is the USB and I cam eject it.