The 3-2-1 Backup Rule, and What It Looks Like in a Real House

The 3-2-1 rule predates the cloud and has outlived most of the products that were supposed to replace it. Keep three copies of anything you care about, on two different kinds of storage, with one copy off-site. It survives because each part answers a different failure, and the failures are not hypothetical.

Three copies covers the ordinary case: a drive dies. Drives die without warning and at inconvenient ages, both early and late. The original plus one backup is two copies, and two copies means a single bad weekend can leave you with none, because the moment a drive fails is exactly when you discover the backup has been silently failing for a month.

Two kinds of media covers correlated failure. Two drives bought at the same time from the same batch, running in the same machine, sharing a power supply and a room temperature, are not as independent as they look. An internal drive plus an external one, or a local drive plus a cloud service, fails for different reasons at different times.

One copy off-site covers the whole-room events. Fire, flood, burglary and a power surge do not care how many drives were in the cabinet. This is also the part that quietly defends against ransomware, provided the off-site copy is not permanently mounted and writable from the machine being protected.

The common mistake is to count a sync service as the backup. Dropbox, iCloud Drive, OneDrive and Google Drive are replication, not backup: they are very good at making every copy identical, which is precisely the problem when the change being replicated is a deletion, a bad overwrite or an encryption run. Most of them keep version history and a trash for some window, which converts them into a partial backup for recent mistakes, but a file deleted longer ago than the retention window is gone from every device at once. Know your service's window before you rely on it.

In a normal household, a setup that satisfies the rule without much ceremony looks like this. The working copy lives on your computer or phone. A local backup runs automatically to an external drive or a small NAS, using something that keeps versions rather than mirroring, so that Time Machine, File History, restic or Borg are better choices than dragging folders across. The off-site copy is either a cloud backup service running in the background, or a second external drive that lives somewhere else and gets swapped every month or two. Rotating two cheap drives between home and a desk drawer at work is unglamorous and works.

Photos deserve a specific thought, because they are usually the irreplaceable part and usually the part living in exactly one place: a phone, syncing to one cloud account. If that account is lost, locked or closed, the photos go with it. A periodic export to a drive you physically own turns a single point of failure into a real second copy.

The part everyone skips is testing. A backup you have never restored from is an assumption. Twice a year, pick a file, restore it somewhere harmless and open it. It takes five minutes and it is the only thing that distinguishes a backup system from a directory that makes reassuring noises.