Attempting to start the game as "Diablo" (not "Diablo (Classic)") results in an error where it requires the CD in the drive. Pressing "OK" loops back to the same error, and pressing "Cancel" brings up an issue that it is unable to load the file "diabdat.mpq".

Running a repair and running as administrator had no effect.

Also getting this issue.

I can't even see the text in the error message, it presents me a blank error box that I can choose between what is probably "Try Again" and "Quit" with the arrow keys and enter button.

Going as someone who still owns the game and found creative ways to make the disc run without using the disc I have an idea for you to try.

Go into your diablo location on your computer. See if you can look for a specific file. Default I get those messages on my PC if I don't have the disc in the drive, but by copying over DIABDAT.MPQ into the root directory of the game kills the CD check.

Not sure this will help, but it's a thought to look into.

I've had to use the HD Bezelbub hack to run this old game on my computer but stupidly enough they tweaked it added some weirdness to it and hacked it up a bit so it's not the same game so this real release here is appealing.

I'm pretty confident I don't have another installation of the game on my PC, so that's not it. I'll submit a ticket.

Same problem here.

I have this problem as well, submitting a ticket

Same problem here. Maybe because i have the original installed?

This worked perfectly, thank you!

I do have the original retail version installed, but by the time I got home from work, I got a new update pushed and whatever it was seemed to have fixed the issue and I'm hacking my way through the Cathedral! Thanks for the really fast turnaround!