Variable Backup path through "subst"?

I am trying to create physically seperated backup folders by mapping a Drive W: to four USB Drives on a Rotation with the “subst” command. (Day 1 W: -> F: | Day 2 W: -> G:| etc.)
When I set the backuppath to w:\urbackup it states the directory cannot be found (err_folder_not_found)
Setting it to F:\urbackup works just fine.

Is there any way for me to get my “subst” Idea off the Ground?