Hey guys. I also have a 20 gb 360 that i purchased back in 2007.
Whenever I load the game, I can watch through the initial cut scene for the single player campaign up until the team jumps into a pair of helicopters and flys off. Then the cut scene starts again. I tried to see if I could just jump into the multiplayer, but as soon as everyone is ready to go play and the map starts to load, I get a disc-read error.
I tried to install the game and play it through the hard drive, but I couldn't even get to the main menu. I just got a green screen. After a minute of that, I get the disc read error.
I also noticed that in the intro video before I got to the main menu screen, u know..where you see a picture of space, then the bungie logo, then some asteroids colliding, followed by the microsoft logo, ect... certain parts of the screen showed static or became very pixilated. Not the entire screen, mind you, but only small noticeable blotches here and there.
I called xbox, went through the usual fix-it tips and nothing worked. I went back to gamestop and replaced the game once, but found that i had the exact same issues with the new game disc.
My thinking is...it can't just be a hardware problem. Every other game that I own works just fine. It's just halo reach. Maybe it's the way they coded it... Anyways.. I hope they come out with a fix for it soon. Preferably a patch update.