iDVD burns a movie but it won't play.

Here's the problem...

The super drive on my macbook pro does not work any more so I purchased an external DVD burner. I then used it to try to burn a DVD from iDVD and it said it had finished but my computer wouldn't mount the movie and nor would a regular DVD player. I tried again and the same thing happened. Then I tried to burn a regular audio and mp3 CD and they both worked fine. I took the external DVD burner to friends macbook pro and iDVD burnt a DVD without any problem. Then I tried to copy that disk using Toast and the same thing happened when I tried iDVD. The movie finished but it wouldn't play. I realize this may not be an iDVD issue but any thoughts would be very helpful!

Thanks,

Cory