UAC is likely your problem. By default, network drives mapped in a user session are NOT mapped into administrative space(or other user space for that matter). You have to enable "linked" connections on the client by setting the following key: (This should work on term server as well)
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System]
"EnableLinkedConnections"=dword:00000001
Once you do that, drives mapped in user space will be available in admin space and your code should work.
The same sort of thing happens if you try to copy from a mapped drive to another mapped or local location where privilege elevation has to occur. Without the linked connections, you'll get a 'path not found' error and the only way to make the copy go is to switch from a named drive to a UNC path. This key fixes that problem as well.
HTH,
Jason