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I'm using the GOG version, but every so often it will bring up this prompt when loading a new area

"There is no disk in the drive. Please insert a disk into drive \Device\Harddisk3\DR3."
Occasionally the DR3 at the end will be DR2.

It gives me three options: continue, try again, and cancel, all of which just close the prompt and let the game resume loading. On some areas it will open that prompt 27 times, and then bring it up several times as I move about the map.

The only thing I can think of that may have anything to do with it was install this

and without that, the game runs slower than molasses in an igloo for me.
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The only thing I can think of is to try installing the Critical Rebuild.