So here comes my first little tidbit.
If you have a classic MacBook Pro Unibody, chances are good that you exchanged the internal DVD drive for a second harddrive and put the DVD into an external enclosure. This is done easy enough and provides lots of bonuses but the DVD-Player-App won’t recognize the external drive.
The reason for the problem is that the Mac wants to address the second SATA port to access the DVD but that is the port you just installed you second harddrive to. Now you have to tell the Mac that the DVD drive sits at SATA port three which is the one in the external enclosure.
You can do this by editing the DVDPlayback in the Folder: /System/Library/Frameworks/DVDPlayback.framework/Versions/A. There for you would have to use a HEX-editor (like 0xED) and change any mention of SATA Port 2 (“496E7465726E616C”) to SATA Port 3 (“45787465726E616C”) which is a pain in the a...
Or, you can just take the already edited version I uploaded here... Your welcome!
Copy the file to /System/Library/Frameworks/DVDPlayback.framework/Versions/A , after copying the old version for backup purposes to a different location. Then you repair the disk permission of your system disk with disk utility and restart the computer. Tada, DVD player should now recognize the external DVD drive.