How would I open a specific DVD drive when it's no longer in use?

Question

Specifically I am using HandBrake to rip/encode DVDs and when one of the 3 drives finishes it sends the file to a .bat which runs in the background and plays a notification to let me know it's done.

I place another DVD into it and add it to the queue. To avoid replacing a DVD that hasn't run yet by accident I'd like to have it open the drive that last finished.

If you're unfamiliar with the program this is how it works:

• When I add a movie to the queue it has the source file open and idle awaiting encoding.
• Encodes the file.
• Closes source and opens my .bat.

I'm hoping there is a way to detect which drive isn't open in the program using my batch file and eject it.