ISO burn - "The disc drive is busy...."

I'm trying to use Disk Utility to burn a bootable ISO file as follows...
1. put blank DVD in super drive - Macbook Pro 2011
2. put ISO file on desktop
3. run Disk Utility
4. open ISO image so it appears on the left side in Disk Utility
5. click ISO image on left side of Disk Utility
6. click "Burn" yellow and black icon
7. observe message in disk window...
" Burn Disc In: SuperDrive"
"The disc drive is busy. Please choose another drive."
8. eject disk, get another new disk
9 repeat from step 1.

It seems to think that the drive is being used for something else, when it is not, and I can't seem to persuade it otherwise. Suggestions?

Thanks again!

Doh - I found that VMWare was sharing the drive with a Windows VM. Funny how you forget things like that after a while.

Although this problem is solved, I would like to know - how do you find out what programs are using specific drives/folders/files in OSX?