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madmatt2185: I got that too. I tryed to change the options for the game, making it windowed or 720P And since that first try I can't even get the game to load, much less tell me the CD isn't there. Just grays out the play buttion and does nothing.
If you get greyed out 'Play' button, check Task Manager for any stray diablo.exe processes and kill them.
madmatt2185: Ok, it was hiding a program still running. Still getting the "Please load CD" prompt though.
Please see my post here and check whether the workaround applies to you: