How do I back up my MacBook Air Computer
My MacAir Air is telling me I have not backed up in a long time. How do I tell it to back up?
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You'll need to connect an external drive to your Mac, then reformat it using APFS (if possible) before using it.
Then, once it's reformatted properly, you'll want to launch Time Machine on your Mac and back it up. Back up your Mac with Time Machine - Apple Support
Time Machine will remind you approximately every 10 days to back up, assuming you don't leave the drive connected at all times.
Once it's backed up, you won't be nagged again about it until 10 days have been exceeded between backups.