I put a bit of work in on this this morning, following Mark's suggestion (msg138197) since that's the "canonical" approach. Unfortunately, it completely fails to work for the most common case: the root folder of a drive! The documentation for FindFirstFile explicitly precludes that possibility.

It looks as though  GetVolumePathName is the way to go. I thought I'd previously found some instance where that failed but, ad hoc, I can't make it fail now. I'll try to rework Atsuo's patch against the current posixmodule.c.