Cornerstone and Titan run as a Windows service. Since Windows services do not generally run under the scope of a logged in user process, they do not have access to mapped network drives (which are available when you log in to the pc).

Instead of using a mapped drive for your server data directory (or virtual folder), use the UNC path name for that mapped drive (Example: \\\\mydocuments\usr\fred\). Also, make sure that the Windows user account being utilized by the Cornerstone/Titan service has access rights to the network share.