Backing up with the 3-2-1 rule

The rule in one sentence

The US agency CISA (then US-CERT) sums it up in its Data Backup Options document like this: keep 3 copies of every important file (the original and two backups), on 2 different types of media, with 1 copy off-site. The rule comes from the photographer Peter Krogh; CISA cites it as recommended practice for both households and businesses.

Why it works this way

Each part protects against a different kind of disaster:

Risk	What helps
The drive in the computer fails	a second copy on another device
Two drives from the same batch fail, or a power surge destroys them	a second type of media
Fire, flood, or the whole flat or office being burgled	an off-site copy
A file deleted or overwritten by mistake	a backup with older versions
Ransomware encrypts everything connected	a disconnected or immutable backup

Syncing is not a backup

Services that sync a folder between your computer and the cloud are convenient, but they do exactly what they say – they sync. If you delete a file, or malware encrypts it, the change is carried over to the cloud. Some services have a recycle bin or version history that helps, but it has a time limit. A real backup keeps older states regardless of what happens to the original.

A practical setup for a household

1. The original – on the computer.
2. The first backup – an external drive or a NAS, automatically (Windows: File History or Windows Backup; macOS: Time Machine). Disconnect the drive after the backup.
3. The second backup, off-site – a cloud backup, or a second external drive that you take turns keeping at a relative’s or at work.

How to choose and connect an external drive is covered in the guide External drives and enclosures.

Test a restore

Every few months, pick a random file and restore it from the backup to a different location. Check that it opens. The worst moment to discover that your backup does not work is when you need it.

NAS in brief

A NAS (Network Attached Storage) is a small computer with drives, connected to your home network. It is handy as a central place for backing up several computers. But be careful: if the NAS has its drives in a RAID 1 array (a mirror), it is still one device in one place – it protects against a drive failure, not against fire, theft or deletion. In the 3-2-1 rule it counts as one copy.