Mapped Network Drive On Same PC Not Writable?

I run some CAD and compiler software that (unfortunately!) uses absolute path names in project files, so I need to access all my files with exactly the same path for my PC and my laptop. Enventually I want to put all files on an external NAS drive, but only have the PC at the moment.So my first thought was create a folder on my PC, say C:X_Drive, map it as a network drive, say X:, and share the folder as X: on the laptop. Then I'd always access files through the mapped drive, and all paths would be consistent, and moving to an external drive should be trivial.

This sort of works. I've set up the Security tab to give everyone full access. I can read and write the folder itself, but cannot write to the mapped drive version on the same PC, I'm warned I don't have permission. I'm an administrator and the only user on the system. I've tried UNC addressing and direct fixed IP addressing when mapping, no joy. Subst doesn't work properly either (drive comes up as disconnected in explorer, but I believe that's a known Windows bug).