How to install macOS on an external drive

Macworld

Concerned about updating your Mac to the latest version of macOS? Perhaps you aren’t sure about running macOS Sequoia quite yet, or maybe you want to test the beta but don’t want to risk messing up your system. You might be concerned about incompatible software or you might just be thinking that you might hate it and not be able to go back to the macOS you are familiar with. 

Luckily you don’t have to install the OS update on your Mac at all. You can install it on an external drive instead.