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- Get an external hard drive that has at least 12 GB of available storage and plug it in your Mac.
- Launch the Disk Utility app and erase your hard drive selecting Mac OS Extended format.
- Open Terminal (Applications > Utilities).
- Paste this command and press Enter:
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sudo /Applications/Install\ macOS\ Catalina.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume
- If prompted, enter your administrator password and press Enter again.
- When Terminal says “done”, your hard drive should have the same name as the installer you downloaded (for example, Install macOS Catalina).
- Open System Preferences > Startup Disk. Choose your bootable installer as a startup disk and press Restart.
- Your Mac should start up to macOS Recovery.
- Make sure your Mac has an internet connection to download firmware updates (you can use the Wi-Fi menu in the menu bar).
- In the Utilities window, select Install macOS.
- Your Mac will then start installing macOS Catalina and will restart when it’s done.