How do I back up my computer?

Backup fails in a predictable way: people back up to a single external drive kept next to the computer, which protects against drive failure but not against theft, fire or ransomware. The 3-2-1 rule exists because those are all real and all happen.

What you'll need

• External drive at least twice the size of your data
• Cloud backup subscription (optional)

Step by step

1. Work out what you'd genuinely miss — usually documents, photos and anything you've made.Applications and the OS can be reinstalled. Your files can't.
2. Buy an external drive of at least twice your data size.Backup systems keep version history, which needs room. Too small a drive means the history gets truncated.
3. On macOS, set up Time Machine and let it run.System Settings → General → Time Machine → Add Backup Disk. It handles scheduling and versioning automatically.
4. On Windows, set up File History for documents, or a full-image tool for the whole system.File History covers user folders. For a complete system image, use a dedicated backup tool.
5. Add a cloud copy for irreplaceable files.This is the off-site copy. It's what protects against theft and fire, which a drive on the same desk does not.
6. Set backups to run automatically, not manually.Manual backups happen for a fortnight and then stop. This is universal.
7. Test a restore: recover a single file and confirm it opens.The only way to know the backup works. Do it now rather than in an emergency.
8. Keep at least one copy disconnected or versioned.Ransomware encrypts connected drives too. Versioned cloud backup or a rotated offline drive protects against it.

Tips

• 3-2-1: three copies, two different media types, one off-site. It's a rule of thumb that survives most realistic disasters.
• Cloud sync (Dropbox, OneDrive, Google Drive) is not backup — if you delete or corrupt a file, it syncs the deletion. Some offer version history, which helps, but it's a different thing from a backup.
• Two external drives rotated, one kept elsewhere, is a cheap and effective off-site strategy without a subscription.
• Label drives with what's on them and when they were last used. Unlabelled backup drives get wiped by accident.

Common mistakes

• One backup drive kept next to the computer — Protects against drive failure only. Theft, fire and flood take both at once.
• Treating cloud sync as backup — Deletions and corruption sync too. Sync mirrors your mistakes.
• Manual backups — They stop happening within weeks. Automate it.
• Never testing a restore — Plenty of backups turn out to be empty, corrupt or missing the important folder. You find out when you need them.

Questions people ask