installing older macOS on external HDD

[crazmxican1]

I was wondering if someone could help me out. I am running Catalina but would like to install an older version os (sierra or el capitan) on an external and be able to boot from it. i have looked up some videos but I'm still having trouble. Any help would be really appreciated.

 

[Boyd01]

See this. You should be able to select the external disk when you install. However, I would never attempt to do something like this unless I had (at least) one good clone of my internal disk, just in case something goes wrong.

How to download and install macOS - Apple Support

Download and install current or previous versions of the Mac operating system on compatible Mac computers.

support.apple.com

 

[crazmxican1]

i DL'ed the installer and used Terminal utility to create a bootable but I'm getting the following message:


sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ macOS\ Sierra.app


Password:


sudo: /Applications/Install macOS Sierra.app/Contents/Resources/createinstallmedia: command not found