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About excluding system files from backups
You can exclude macOS system files and apps from a Time Machine backup, but it is not recommended. Excluding macOS system files saves space, but makes it harder to restore your entire system if your startup disk is damaged and must be repaired or replaced.
macOS system files include the System folder and the apps that come with your Mac, such as Safari, Mail, Messages, iTunes, Dashboard, and others.
To exclude items other than macOS system files, open Time Machine preferences, then click Options. To open Time Machine preferences, choose Apple menu > System Preferences, then click Time Machine.
Open Time Machine preferences for me
If you need to restore your macOS system, and you haven’t backed it up with Time Machine, see Reinstall macOS.