At the “macOS Utilities” screen, choose “Disk Utility”.Immediately hold the OPTION key until the boot selection menu appears on the display, then choose the “Install MacOS Catalina” drive at the boot menu.Connect the MacOS Catalina installer drive to the Macs USB port.Complete a full Time Machine backup before beginning – do not skip making a full backup.Do not proceed unless you are comfortable removing all data and have backups made. Warning: This erases everything on the Mac. Failure to have sufficient data backups may result in permanent data loss, so proceed at your own risk. All personal data, apps, files, and anything on the Mac is erased in this process.

It’s important to remember that a clean install will erase all data and everything on the Mac. Have a complete backup of the Mac and all important data made, use Time Machine to backup or another thorough backup method of your choice.

Have a MacOS Catalina boot USB installer drive already made and ready to use.

The Mac must be compatible with macOS Catalina.

Be sure you have a complete backup of your Mac and all important data made before proceeding.Ī clean install of MacOS Catalina 10.15.x requires the following: Clean installs are not recommended for most Mac users and because the hard disk is erased it has potential to cause permanent data loss, so this is really only appropriate for advanced users with a compelling reason to format their Mac and start over, or if someone is selling a Mac or transferring ownership, or otherwise.

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