Installing macOS Mojave from Big Sur

I ran into a sort of chicken-and-egg problem. I wanted to use a 32-bit app temporarily, but that doesn’t run on Big Sur, because Big Sur doesn’t support 32-bit apps. So I wanted to boot Mojave from an external disk. I downloaded the installer, but the installer is too old to open on Big Sur.

Ingredients:

• Mojave installer, downloaded from Apple
• Empty USB
• Empty external disk

Recipe:

1. Plug in both devices
2. sudo /Applications/Install\ macOS\ Mojave.app/Contents/Resources/createinstallmedia --volume /Volumes/USB
3. Boot up USB
4. Install from USB onto external disk
5. Boot up from external disk