When content is unavailable, such as halo 3, or "disk is unreadable" , try this out.
When my friend brought this up this we were having trouble with my halo dis.
Step 1) Get a banana
2) Peel the banana, and rub the inside of the peel gently on the bottom side of the disk (The side without the graphic)
3) Spread the goo with your shirt or other cloth
4) Wash off with water
5) Dry it off nicely
This has not been tested on anything but the Xbox 360
Edit: My other friend tried this and it didn't work- Reliability not that great apparently. But no harm in trying.
Edit2: Edited with much better grammar and more clear steps. Enjoy.