Lea-Nikkaya:
If you want to play Halo 2 12 level here is a tut if you haven't already used
Fix for Level 12
Play the game until you get to the gravemind level, as the level tries to load you will get the message "Sorry, insert you original Halo 2 disk"
(inserting either the original or any other disk does nothing since the drive is never accessed).
close and restart the game and let it get to the main menu screen.
hit the windows key on the keyboard to minimize the game and open process monitor, let it run and monitor processes (the list wil fill rapidly).
Click on the Halo 2 icon in the taskbar to re-maximize the game.
click on "Campaign". Do not resume, select new and then select the gravemind level.
Let it load (It will take awile because process monitor is slowing it down). Don't wory about missing the intro Video, once the level loads, it can be restarted from within the level.
Once it loads and starts, (wait for the intro to finish) minimize with the windows key again and close the Process monitor.
Re-maximize the level, hit the escape key and select restart the level (if you want to see the intro again). The level can be reloaded from checkpoints as long as you are within the level, if you quit the level for any reason, you will have to run the Process Monitor again while the level loads.
You can load a saved checkpoint in this manner, so you don't have to complete the level at one sitting.