It doesn't matter whether you map the drive or not, whether you have permitions on the network drive or not. Oracle simply doesn't accept network-maped drives as being capable of storing database files.

I don't know NT (or Oracle's) internals enough to tell you the exact reason, but frankly I don't blame Oracle to refuse network drives as hosts for database files - there are just to many possibilities that something might go wrong with maped drives, IMHO the whole concept is simply to risky for RDBMS.