Windows:
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Use File History or Backup and Restore:
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Go to Settings > Update & Security > Backup
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Or use Control Panel > Backup and Restore (Windows 7)
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Mac:
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Use Time Machine:
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Plug in an external drive
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Go to System Settings > Time Machine
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Select the disk and enable automatic backups
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For more control, consider third-party tools like Acronis, Macrium Reflect (Windows), or Carbon Copy Cloner (Mac).