Can a mac boot from an external drive, if the external drive is running an older version of macOS? In general, can a make that has a newer version of macOS on it boot from an external hard drive that has an older version of macOS on its main drive?
For example, could I boot High Sierra from an external hard drive on a mac that has Catalina installed on its' main drive?
Why? I have a mac desktop running Catalina that's a work machine. I'd like to use the hardware for personal use, and run macOS using an external hard drive, using an older version of macOS that I've purchased myself, but when I try, I see the &quot;Prohibited&quot; sign on the drive when I try and boot from it.
I tried to use the external drive with a macbook running El Capitan, and it worked fine
EDIT: The model is &quot;iMac Retina 4k, 21.5, 2019&quot;
A 2019 Mac is much too new to be able to boot High Sierra. In general, a Mac cannot run any macOS version whose release date is earlier than the release date of the Mac.
If you have a specific need to run an older macOS version (but no older than Lion, or at most Snow Leopard Server), you can do so in a virtual machine using software like Parallels or VMware Fusion.