Have just finished the Infernal Machine but didn't have to use the 2nd CD. Can anyone tell me what was the point of the 2nd CD?
Very puzzled.

When you first installed the game, you must have done a Complete install. When actually playing the game, you could insert either CD just to prove that you own the game. If you had done a Typical install, it would have installed less on your hard drive...and you would have to use the second CD after completing the first 8 or so levels. (The complete install is better because it loads faster.)