The 3-2-1 backup rule, explained simply

The rule

The rule is recommended in “Data Backup Options”, a paper produced for US-CERT, part of the US government. The paper credits it to photographer Peter Krogh's book on managing digital files, and says that saving just one backup file may not be enough to protect your information. To improve your chances of getting lost or damaged files back:

• 3: keep 3 copies of any important file: 1 primary and 2 backups.
• 2: keep the files on 2 different media types, to protect against different types of hazards.
• 1: store 1 copy offsite, for example outside your home or business.

Why each number matters

Three copies, because backups fail too. A drive can die on the same day you need it.

Two kinds of storage, because different kinds of storage fail in different ways. The paper points out that if your backup sits on the same internal drive as the original and that drive is damaged, stolen or corrupted, you could lose both.

One copy somewhere else, because fire, flood and theft take everything in the building.

What it looks like at home

1. Your computer holds the original. That is copy one.
2. An external drive holds a backup. That is copy two, on a second kind of storage. On Windows, File History can keep it up to date for you.
3. A cloud backup holds another. That is copy three, and it is the one stored somewhere else. Windows Backup can do this with a Microsoft account.

Set both up with our guide, how to back up a Windows PC.

Three mistakes to avoid

1. Leaving the external drive plugged in all the time. The paper's advice is to remove the drive from the computer when the backup is complete. Keeping backups physically separate protects them from online attackers and power surges.
2. Never testing. Restore one file every so often to prove the backup works.
3. Backing up only once. A backup from last year protects last year's files. Back up on a regular schedule.

Where a plan fits

Every Tech Support Bundle plan includes cloud backup for the covered computer: 50, 100 or 250 GB, depending on the plan. That covers the copy that lives somewhere else. Add an external drive at home and you have 3-2-1. See backup help.