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I’ve tried a few times now but I get a message ‘you need to update to a later os’ when I try to boot up from the external.

It would seem that Catalina doesn’t like it. I still want Catalina installed but I need Mojave as an external boot drive.

Any ideas anyone?

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    Are you trying to run the Mojave installer from the Catalina os? There’s no issue having both bootable, so I suspect there’s something we’re missing in your setup.
    – bmike
    Commented Dec 23, 2019 at 21:18
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    You will need to make a bootable installer, boot from that ant THEN install Mojave on the external drive, or are you already doing that. Commented Dec 23, 2019 at 21:34
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    Yes I’ve already done that but it won’t boot. I have a new MacBook Pro running Catalina. I installed the installer on an external drive and when I boot from it I get a message saying ‘The version of macOS on the selected disk needs to be reinstalled.
    – kowalski
    Commented Dec 23, 2019 at 21:56

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I have a new MacBook Pro running Catalina.

macs require you to run whatever macOS was shipped with the machine or a latter macOS. So, you bought a new mac that came pre-installed with Catalina which means you cannot run Mojave on it. You could get a virtual machine and run Mojave in the virtual machine. You can get Parallels and run Mojave. You can try the free virtual box, but expect some difficulties install macos.

If you are still in the free return period, you can return your machine. Look for a new machine with mojave on it.

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  • I might add as an addendum—Mac models that never came preinstalled with an older OS can't run the that OS (unless you use hacks and are very lucky). But sometimes Apple updates the OS of the same model mid-gen. You can downgrade those. BTW, I've discovered VMWare Fusion is much better than Parallels at running macOS guests. Commented Dec 24, 2019 at 1:46
  • Thank you, very helpful. Looks like Sim stuck with Catalina for the moment
    – kowalski
    Commented Dec 25, 2019 at 22:30

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