by TIM BUEL
Here is a guide on how to create a bootable external Mac hard drive to run macOS from an external hard drive.
There are quite a few reasons for why you may want to be able to have an additional startup drive for your Mac. One of the main reasons is backup of your current system. Another is creating a drive for testing your Mac’s hardware. No matter the reason, having an additional bootable option on an external hard drive is definitely a good tool for any Mac user to have.
See directions here