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When I try to open the Mojave install app on my computer with Big Sur, it says that the operating system is too old and so it will not let me open it in order to install it on the external usb drive.

Additionally, the alternative method - restoring Mojave onto the external drive is not possible because the Mojave install file is an app and not a disk image.

How can I proceed to create a bootable copy of Mojave on my external USB drive considering I am on a computer with Big Sur and the Mojave download is an app file which will not open and it is not a DMG file which I would otherwise be able to restore onto the external USB?

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Follow the createinstallmedia directions here: How to create a bootable installer for macOS

For the sake of completeness, here’s the command you would run in Terminal to create the install media for Mojave (it’s slightly different for each OS version):

sudo /Applications/Install\ macOS\ Mojave.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume
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  • Ok, it almost worked, I now have USB key that shows "Install macOS Mojave" but when I boot my laptop while holding the 'option' key, only the original laptop volume appears as bootable.
    – Andrew Richmond
    Commented Feb 10, 2021 at 22:55
  • Did you reformat the drive as an HFS+ (Mac OS Extended) volume before running the createinstallmedia command?
    – pion
    Commented Feb 10, 2021 at 22:58
  • Mac OS extended journaled w/ guid partition map scheme
    – Andrew Richmond
    Commented Feb 10, 2021 at 22:59
  • What model & year is your computer?
    – pion
    Commented Feb 10, 2021 at 23:00
  • 2017, 12 inch macbook
    – Andrew Richmond
    Commented Feb 10, 2021 at 23:00

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