Path Upload folder

Good morning all,
I want to change upload folder from IIS Server to another file server.
Under Fields - Properties - Edit as - File/Image ...
I insert absolut path on IIS Server sample: C:\inetpub\wwwroot\Projekt\file
It works fine.
But ..
If I try UNC Path
\\server\upload\file
It doesn't work.
Virtual path and mapping drive doesn't work, too.
Can anyone help me ?
Thank you - Martin