Halo 4 Disc 2

Disc 2 is the multiplayer component of Halo 4.

You do not need disc 2 to play Halo 4, only Disc 1 is needed.

Frankie did confirm Halo 4 is 2 discs in a interview like last week or so. 1 disc is campaign and the other is multiplayer. Luckily you can install the 2nd disc and play the entire Halo 4 experience off of the 1st disc. It works like the Forza situation.

By deleting everything I literally mean, if you go into Storage, there should not be Halo 4 in that listing. Delete _everything_. Take the disc out, delete Halo 4, save files, title updates, _everything_. Make sure there's no disc in the console, and double
check to make sure it's completely gone in Settings -> Storage and scrolling down to Halo 4. Clear your HDD cache a few times.

Only after you're sure that Halo 4 does not exist on your console, no trace or mention of it, put Halo 4 Disc 1 in the console, start the game as you would normally, DO NOT INSTALL, do not do anything. Play the game. Start the campaign. Save and quit. The
game should ask you to insert disc 2 before you can do anything anyway. Once you're in a campaign mission, save and quit, and then you can go ahead and install disc 1.

This is just shoddy work on Microsoft/343i's part. If you install Disc 1, and not have the game create disc 2 cache, it'll freak out. It checks to see if you have Halo 4 installed. It doesn't check whether it's disc 1 or 2. It expects disc 2 cache, and sees
disc 1 instead on your HDD, and it freaks out. Follow the instructions I mentioned. If that doesn't work, then I'm not too sure what's going on.

Have you tried what I suggested? Delete _everything_ Halo 4, except your save files. Put Disc 2 in, start the game from Disc 2.