Quick question about Disc 2
I've completed the original 1000G for Halo 3 and want to buy the DLC packs now but someone told me that the 2nd disc of ODST contains them all and i should buy that instead. Will i be able to earn every map pack achievement for Halo 3 just by playing ODST multiplayer? How does that work? Do i have to install the 2nd disc or is it a download key for marketplace? I ask this because i'm concerned about preowned versions.