You want to change the path of what to where?
Remember an external USB drive will keep changing it's drive letter if it finds other drives occupying it original drive letter, so what exactly do you want the external drive to be used for, storage, yeah, but of what? My Documents can be transferred straight by right clicking on it and changing it's address in the properties.
Program Files for newly installed programs can be given a new path , to the external HDD by changing their installation directory during the install process itself.
So what is it that you exactly want sent to the external HDD?