I have two domains, PRIMARY and EXTERNAL. EXTERNAL has a one-way outgoing trust to PRIMARY so that PRIMARY's users can authenticate in EXTERNAL. Both domains have Windows Server 2008 DCs running at the Windows Server 2003 compatibility level. PRIMARY users are generally stripped of their authentication privileges in EXTERNAL (including PRIMARY domain admins) but the few users with explicit access get the authentication privileges granted.
The EXTERNAL domain controller has a share called Projects on which everyone has full access. The folder is then locked down with ACLs to only allow a few of EXTERNAL's administrative groups. A few levels down in this folder hierarchy, there is a folder where a user (TESTUSER) in PRIMARY is given modify access. The UNC folder path to this folder is
When PRIMARY\TESTUSER is logged into a PRIMARY domain-mapped computer with Windows 7, trying to go directly to the path does not work. ("[unc path][new line]The specified path does not exist. Check the path, and then try again.") However, mapping the folder with the exact same path as a drive works, and the appropriate privileges apply (files can be created, etc).
What can I do to enable the UNC path to work? What causes the discrepancy?