Microsoft IIS will not allow a mapped drive to serve as a virtual directory. You can use a UNC designation if you want, but this is prone to errors and problems.
See: http://support.microsoft.com/default.aspx/kb/257174
This is problematic because drive mappings are based on credentials of users who are currently logged in, while Microsoft IIS has to work under the permission of a specified user, typically ASPNET or IUSR_MACHINENAME, both of which are local users to your machine and do not have permission to access network drives.
Iron Speed Designer will generally handle files located on mapped network drives. However, Visual Studio 2005 will display errors complaining that various objects "is not a member of" the page.
Creating a Virtual Directory for Your Application
Deploying Applications with the Deployment Wizard
Manually Deploying an Application to a Production Server