The second Halo 4 game disc features the game's multiplayer components and must be installed in order to be played, Microsoft has told Joystiq.
Microsoft says that Halo 4's second 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'".
This means Xbox 360 4GB owners will need to purchase an additional storage device - a USB memory stick will suffice.
This isn't the first time Microsoft has forced content to be installed, having previously done something similar with Forza 4.
Halo 4 will be released worldwide on November 6. The game has already leaked onto torrent sites.