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.