Posted by: AJ Spyder
Will the achievements that you have gotten on the original disc be on that second disc?It's classed as Halo 3 Multiplayer, so it will still work as thought it was the Halo 3 disk you're playing.
Can you upload the second disc data on to your hard-drive so you wont have to put in the disc?Nope. Unfortunately, Bungie can't fix this problem for the Halo 3 Multiplayer stuff as it was created prior to NXE, thus isn't supported that well. Besides, even when you install a game to your HDD, you still need to put the disk in to "clarify you own it". Which I think it stupid. So yes, the Halo 3 Multiplayer content will still take longer to load from the HDD than from the disk.
Will disc 2 have custom games, forge, theater, and spartan/elite customization?Yes.
Can you still go in to MM with someone who is using the original halo 3 disc even if you are using ODST second disc?Again, since it's still Halo 3, yes, they are compatible.