I have been interested in Halo 3: ODST not because of the single player, but the separate Halo 3 multi-player disc. My plan was to install the game disc for the multiplayer from a borrowed copy of ODST in order to obtain the extra map packs that I have yet to download myself. After minutes of installing the disc and swapping out my own copy of Halo 3 with the ODST multiplayer disc, I was disappointed that I couldn't play Halo 3 "from the hard drive" in the game library on the 360's dashboard. When I fired up the game I was equally bummed to find that my scheme didn't work at all. The conclusion of my little experiment was that installing a disc does not allow that data to be read by a "different disc".
Since the Halo 3 ODST and the Halo 3 we're the same game, but not the same kind of game disc, thier data cannot be read by each other. This is why Bungie put all the downloadable content onto the multiplayer disc for ODST; because existing content for Halo 3 could not be read by the ODST disc. There is an exception, but not in this case, of discs able to be read by different copies of the same game. My copy of Elder Scrolls IV: Oblivion which is a game of the year edition, has a separate disc that, when installed following the prompts given when the disc inserted the content, is available for any copy of Oblivion that is placed in that 360. Notice that I said 'follow the prompts'. The process of installing the Oblivion disc is different than what I tried to do with the borrowed copy of ODST.
Now we run into a situation that few videogame publishers have developed a solution for. How do you give players without internet access to downloadable content the ability to play the DLC for their single player games? Earlier solutions we're to resell the same game with the added content i.e. Super Street Fighter II. Another attempt was to sell the DLC as a separate disc, either by itself or with original copy of the game, and is installed and played with an original copy. Rock Star's solution for the Grand Theft Auto IV DLC was to sell it on a separate disc that could be played by itself, but it only has the extra content and not the original game content. Fortunately not every gamer has to worry about where there extra content is coming from. Most home video game consoles are hooked up to the internet and receive constant updates and purchased content just fine.