How to back up an iMac to an external hard drive
How do I back up my iMac on to an external hard drive
iMac (2017 – 2020)
Open System Preferences or System Settings and set up Time Machine, or use a product such as Carbon Copy Cloner.
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Everything you need to get started: Back up your Mac with Time Machine - Apple Support
How to back up an iMac to an external hard drive