I was using, I think v6.3 and it had no problem recognizing SUBST drive P:, which I can see in my Windows Explorer. Once I loaded v6.6.4, Notepad++ acts like there is no drive P: (in my example).
NEVER MIND! I trued deleting and re-adding the SUBSTed drive and now it works.
Unrelated to N++. If you SUBST a drive using user X, then user Y won't be able to see it. This holds in the specific case when one of X, Y is administrator...