How to install multiple copies of macOS (perhaps on external disk)?

by KaiserS   Last Updated September 11, 2019 20:01 PM - source

I'm having a lot of trouble installing multiple (older) versions of macOS on an external hard disk. I need them to test my software for users who refuse to upgrade!

Apple has made this extremely difficult, and I can't find a walkthrough anywhere.

Has anyone had success with this? Perhaps using a VM?

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