I experienced that whenever I inserted a DVD on my Ubuntu-driven HTPC and attempt to play it using XBMC I had a black screen without any visible menu or sound. When pressing the stop button, my computer froze completely. This bug is filed here:
After searching and testing around a while I found an interesting option in the XBMC Video Settings under the DVDs tab. There is an option called “Attempt to skip introduction before DVD menu”. I switched it on and suddenly everything works. No more black screen and no more freezing when pressing the stop button!
My second guess would have been the solution described here:
Apparently disabling auto mount solves this problem as well. So if the first solution doesn’t work for you try the latter one.