Create bootable clone of Mojave startup disk to another internal drive


Hey all - I am on Mojave, MacPro 5,1. want to create an exact copy of my startup disk (Bay 1) and I want to clone it to another internal drive (Bay 4). It appears that Carbon Copy Cloner will only create the clone to an External drive. It keeps calling it a Dry Run when I attempt to clone from one internal to another internal and then doesn 't give an option to proceed. Just another dry run. I have reformatted the drive in Bay 4 according to instructions. It has enough storage space
I don't have an external drive to use.
What's the issue that won't allow me to do so? This is frustrating!