WHAT IS THE "DIRTY DISC" ERROR I KEEP
GETTING WHEN I JOIN A GAME AND HOW CAN I FIX IT?
When joining a game, you might receive a Dirty Disc error. This does not
mean your disc is scratched or dirty. This is a known HALO bug.
Follow These Steps To Prevent The Dirty Disc Error
Before Joining A Game:
-Step 1: After Selecting your Player Profile, you will be on the
"System Link Games" screen.
-Step 2: Instead of pressing the A Button to Select A
Game, you will need to press the Y Button to Create A Game.
-Step 3: On the "Select Map" screen, select any map you wish
and press the A Button.
-Step 4: On the "Select Gametype"
Screen, select any gametype you wish and press the A
Button.
-Step 5: Doing those steps will bring you to the "Waiting for
another Xbox console" screen. Press the B Button 3 Times to back up to the
"System Link Games" screen.
-Step 6: Select the game you wish to play and press the A Button.
-Step 7: Wait for the Host to start the game and everything will work
fine.
source: XBconnect faq