Here's how you find the process that holds your DVD drive and eject it to your heart's content: First, simulate it. Stick a disk in your DVD drive, open up a terminal, and mount the DVD drive:
# mount /media/cdrom
# cd /media/cdrom
# while [ 1 ]; do echo "All your drives are belong to us!"; sleep 30; done
Now open up a second terminal and try to eject the DVD drive:
You'll get a message like:
umount: /media/cdrom: device is busy
Before you free it, let's find out who is using it.
# fuser /media/cdrom
You see the process was running and, indeed, it is our fault we can not eject the disk.
Now, if you are root, you can exercise your godlike powers and kill processes:
# fuser -k /media/cdrom
Boom! Just like that, freedom. Now solemnly unmount the drive: