The game was made back before games had full installs to the hard drive. So a lot of the game was never installed on your hard drive and remains on CD2. You'll play using CD1 exclusively for a while. But after reaching a certain point in the game, it will ask for CD2. After that point you may have to start the game with CD1 for the copy check and then immediately switch to CD2 to play the game (a nuisance).