Subst.exe is used to associate a path with a drive letter.
If you use Subst.exe to connect to a network resource using a Universal Naming Convention (UNC) path, the command runs successfully, but when you try to gain access to the new drive letter you receive an error message similar to:
The filename, directory name, or volume label syntax is incorrect.
To work around this behavior, use the NET USE command to connect to network resources instead of the SUBST command. For example, the command
net use * \\fileserver\share\path1\path2
Results in the next available drive letter being mapped to the root of path2.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed at the beginning of this article.