What might cause "subst" to say "Invalid Parameter - D:"?

If you get the following error message when running subst:

Invalid parameter

I would suggest initially checking if the drive you are referencing is already in use by the system. To check, in the same command prompt as the one you you are running subst run the command:

 wmic logicaldisk get name

The output of this will be something like:

Name    
C:
D:
E:

Note: For a drive that is already created with subst you will get the error:

Drive already SUBSTed