A failing disk, a laptop stolen on a train, coffee spilled on the keyboard, ransomware, a file deleted by mistake: the causes of data loss are many and almost all mundane. Backup is the only measure that covers them all. The 3-2-1 rule is the simplest way to organise it without leaving gaps.
The rule in one line
- 3 copies of your data: the original plus two backups.
- 2 different media: for example the computer's internal drive and an external disk, or a NAS.
- 1 copy off-site: in another physical location or in a cloud service.
The logic is that each copy protects against a different kind of incident. The external disk saves you if the computer breaks; the off-site copy saves you if a theft, flood or fire hits both the computer and the disk you kept next to it.
Syncing is not backing up
This is the most common misunderstanding. Services such as cloud-synced folders are very convenient, but they replicate every change in real time, including the wrong ones. If you delete a folder, it disappears from the cloud too. If ransomware encrypts your files, sync uploads the encrypted versions.
A real backup keeps multiple versions over time: you can go back to a file as it was yesterday, last week or last month. Some sync services offer version history and a recycle bin with retention periods: that's useful, but you should check how long it lasts and whether it covers every file type.
What to back up
Not everything has the same value. Take a quick inventory:
- Irreplaceable: photos, family videos, personal documents, work archives, accounts, projects. Top priority.
- Rebuildable with effort: program settings, browser bookmarks, document templates.
- Easily recoverable: programs and the operating system, which you can reinstall.
For an individual, protecting the user folders is often enough. For a small office it's worth adding a full system image of the main computers, which lets you get going again in hours rather than days.
An example for a family
- Copy 1: the files on the laptop.
- Copy 2: an automatic weekly backup to an external disk using the tool built into the operating system (File History on Windows, Time Machine on macOS). The disk is plugged in for the backup and then unplugged.
- Copy 3: a cloud backup service with version history, or a second disk kept at a relative's house and swapped every month.
An example for a small office
- Copy 1: the data on the shared server or NAS.
- Copy 2: a daily backup to a second NAS or to rotating disks, with versions kept for several weeks.
- Copy 3: an encrypted replica to a cloud service that supports immutability, meaning copies cannot be changed or deleted for a set period, even with administrator credentials.
Immutability or a physically disconnected medium matters because people attacking a network often hunt for backups to destroy them before they strike.
Encrypt your backups
An external disk holding all your documents is a valuable target if it gets lost. Turn on the encryption offered by your backup tool or operating system and keep the encryption key in a safe, separate place, for example in your password manager and on paper. Without that key, not even you will be able to restore the data.
Test the restore
A backup is only worth as much as your ability to restore it. At least every few months:
- pick a few files at random and restore them to a different folder;
- open them and check they're intact;
- check the date of the last successful backup;
- for business systems, occasionally test a full restore on a test machine.
Many people discover the backup had stopped months earlier on the very day they need it.
Automate
Human discipline is the weak point. A backup that depends on you remembering to do it will get skipped. Set up automatic schedules and failure notifications. If you use an external disk you plug in by hand, put a recurring reminder in your calendar.
Mistakes to avoid
- Keeping the backup disk permanently connected to the computer.
- Storing the "off-site" copy in the same drawer as the laptop.
- Relying only on cloud sync with no versioning.
- Not knowing where the backup encryption key is.
- Never testing a restore.
In short
- Three copies, on two different media, one of them off-site.
- Sync replicates mistakes too: you need a backup with multiple versions.
- At least one copy must be disconnected or immutable.
- Encrypt your backups and keep the key separately.
- Automate, and test restores regularly.