How to install mac os from external hardisk to macbook air
How to reinstall macos from external disk to macbook air?
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How to create a bootable installer for macOS - Apple Support will instruct you how to use an external drive to install mac OS. The external drive has to be bootable format.
Please note: You don't need a bootable installer to install or re-install or update mac OS. There are other ways to do these things.
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