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Yes, and I think Starforge looks for Starcraft in .../Program Files/Starcraft
Starforge uses the registry system.
You see when you install, the installer will set up a Registry key on where it was installed. This key lets it know where it is installed(for upgrades), other ones to show that it is installed, if brood war is installed.
Starforge just gets the key that points to where Sc is installed. If that key is gone, im not sure what error would come up, but I would think it would ask you to manually put the directory in.