Halo 4 code leaked online recently and netted the pirates some Xbox Live perma-bans for their trouble. After shots of Halo 4 code appeared featuring a second disc, gamers began to wonder what it was for. Microsoft has now put speculation to rest.
Microsoft stated way back in the summer that Halo 4 would require 8GB of free hard drive space to play.
The publisher recently revealed to Joystiq that the second Halo 4 disc is that 8GB install, which covers the game's multiplayer component, stating that the disc "contains data that will need to be installed on an Xbox 360 hard drive or compatible flash storage device with at least 8GB of available memory in order to access 'Halo 4's' extensive suite of multiplayer features."
There we go, mystery solved.