It’s important to back up your Mac mini regularly to keep your files safe. You can back up your apps, accounts, settings, music, photos, movies, and documents using Time Machine, which is built into your Mac. Time Machine doesn’t back up the macOS operating system.
Decide where you want to back up your files. Time Machine can back up to an external storage device connected to your Mac mini, as well as other supported storage devices. For a list of devices supported by Time Machine, see the Apple Support article What to do before you sell, give away, or trade in your Mac.