IIS Virtual Directory map to network drive (with drive letter) but fail to create file

I am using C#/ASP.NET, IIS6 on Windows Server 2003.

1. Map the data server shared folder to the WebServer with driver letter V:\

2. On the WebServer, IIS created a virtual directory and pointed to 'local location' with V:\ . Since there is no 'Connect As', not sure which USER will be used

3. In my WebMethod, I want to create file on the shared folder with FileStream.Write().

   I got IO Exception on the action, any hint?

P.S. I have added ASPNET/NETWORK SERVICE on the data server shared folder.

thanks! Gavin