How can I run an older OS from an external drive other than Big Sur I have installed on my Mac mini 2018?- Say Catalina.
Mac mini, macOS 11.0
In case you meant how you boot from the external drive once you've installed macOS to it, you restart, hold the Option key and choose the external drive.
You would have to create a bootable installer of Mojave or Catalina. Boot from it and install to the external drive.
How to create a bootable installer for macOS - Apple Support
Ok once I create the bootable installer, do I restart the Mac to prompt then me to enter from the external drive our will the restart automatically boot from the USB drive?
It explains how to use the bootable installer on the bottom of the page I linked.
Is it possible to run an older OS on my Mac mini 2018 or even to dual boot?
