Short answer: Use a suitable backup drive and confirm that a backup actually completes.
Try these steps
- Connect a suitable backup drive. On current macOS versions, open System Settings → General → Time Machine → Add Backup Disk.
- Choose the backup disk and follow Apple’s setup instructions. Read any erase or format prompt carefully before proceeding.
- Let the backup finish, then check the latest successful backup and test finding a file.
When to ask for help
Stop if the drive contains files you still need. Keep another protected copy of irreplaceable data.
Learn more
Learn more: read this article — Apple: Back up and restore with Time Machine. Menu names and available features can vary by device, software version and account type.
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