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I need to add a Mojave partition to my native Catalina iMac 2020, 27", T2. To date all attempts to boot from a Mojave external install drive have failed. "Secure Boot" is set to "No Security" and "External Boot" is set to "Allowed". Catalina just shows a "No Entry" sign after selecting the Mojave Installer drive. I know this machine will support Mojave. I am able to boot exclusively from a Catalina external install drive. If I wipe the computers internal hard drive from a Catalina external install drive will I be able to access the machine and install Mojave? If so how do I proceed? Any other possible scenarios. What about target disk mode?

Thanks again.

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  • Have you tried booting the Mojave installer drive from the Mac Startup Manager?
    – David Anderson
    Commented Jan 2, 2021 at 3:34
  • Yes, a "No Entry" sign appears after selecting the Mojave Installer drive and the computer shuts down. I 'm able to boot from a Catalina external install drive only.
    – zerogvfx
    Commented Jan 2, 2021 at 3:55
  • Why do you believe a 2020 Mac will boot from a 2018 OS? That's generally not the case.
    – Tetsujin
    Commented Jan 2, 2021 at 9:38

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Your iMac shipped with Catalina. Generally, Apple does not support using an OS older than what the device originally shipped with. Yes, there are the occasional exceptions. Unfortunately, your model is not one of them.

If you really need to run Mojave on your system, then you should probably consider running it within a virtual machine.

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  • Basically the same topic, see also apple.stackexchange.com/questions/406019/…
    – Johnmager
    Commented Jan 2, 2021 at 16:45
  • Upon further investigation a virtual machine does seem to be the only current viable option. Many thanks.
    – zerogvfx
    Commented Jan 3, 2021 at 0:15
  • If you are going to consider using a virtual machine, the does the OS have to be Mojave? Older versions of macOS usually preform better in virtual machines.
    – David Anderson
    Commented Jan 3, 2021 at 0:18
  • Thanks for the heads up. One concern is with the new hardware introduced in the 2020 iMac’s, ie the Radeon Pro 5700 XT 16GB graphics card. Mojave may have a better chance of being hardware programed to support the new hardware and provide the new drivers. I will test High Sierra first to see if it is a viable solution.
    – zerogvfx
    Commented Jan 8, 2021 at 20:50

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