Might be becaue you've turned off Auto-Insert Notification on the CD drive, so since Window's hasn't realised you've changed the CD, Diablo thinks you've still got some other disc in the drive. You could always try rebotting with the CD in the drive to see if that's the case, although that's a short term solution.

A longer term solution, which I'm not sure works on all PCs, but at least on mine, is to turn it back on, then if you don't like how it actually autoruns like I do, use TweakUI to turn it off for Data CDs only - it still seems to pick up the fact the disc changed now, just doesn't autoplay it.