From LedHed's Wiki
I came across an issue with UAC today.
UAC had been turned off, but I recently turned it on. I opened an elevated command prompt (RunAs Administrator) and noticed that I couldn't access my mapped drives.
I found the following solution. Create the following registry key and reboot.
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System EnableLinkedConnections = 1 (DWORD)
Reference
http://www.winhelponline.com/blog/mapped-drives-are-not-seen-from-elevated-command-prompt-in-vista/