Thanks for the note. Do you mean the checkbox that says "Install "Toast It" and "Mount It" desktop menu items" ?
Anyway, I unchecked it, restarted, and still have the problem.
By the way, the problem exists whether or not Toast is running.
Again, I can insert the DVD into another computer without any trouble, and I can mount DVDs burned previously using my Mac's built-in burning without any trouble either.