I had the same problem with my 360 trying to load the new Halo 3 map pack. I am sorry to tell you it is a hard drive problem. You can format your drive and start over and it will probably work but you more than likely have a bad spot on your drive like I did and will run into this problem again if you don't get a new one. If you are cheap and don't mind losing some of your saved games to access the online then go to the system tab on your dashboard, select memory, and format your hard drive. There is a code that you will have to enter to format. Just follow the on screen steps, it's not too hard. Be sure to download the map pack first so it is more likely to find a good spot on your drive to reside.