When trying to map a UNC path to a drive letter in the RD, you get a message saying that "an extended error has occurred."
Some NAS or SAN devices using SMB 2.x do not respond with a signed error response as required by SMB 3.0.
1. Open a Command Prompt window in the RD (under the Analyze menu).
2. Run regedit.exe.
3. Browse to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\LanmanWorkstation\Parameters
4. Set RequireSecureNegotiate to 0.
5. Close the Registry Editor and re-try mapping the share.
For more details, please refer to the following Microsoft article: http://support.microsoft.com/kb/2686098.