How do you create a bootable drive on an external disk? I have a MacPro that i want to keep on Mountain Lion, but I'd like to be able to startup off of the latest MacOS.  Is there a way to create a bootable drive that uses High Sierra?
You can simply connect an external drive, open the macOS installer, and install on the external instead of the internal.

Or in the case of a MacPro, just put additional internal drives in there and install on one of them. It's very easy to do. You can have as many different disks and OS versions as you want. My own Mac Pro has 10.7 on one drive and 10.13 on another.