>When a built-in function like DIRECTORY(), FILE() or FCREATE() is passed a UNC-path that does not (no longer) exist, it takes a very long time to tell you that. Is there an API function that can tell this much quicker?

Did you try it with drivetype? Though it may not work with UNC.

http://www.berezniker.com/display/VFP/Enumerate+Available+Drives