(5S Step 1 — Sichern) What is the 3-2-1 backup rule, and why is each number there?

Keep 3 copies of your data, on 2 different media, with 1 copy off-site.

* Each number defends against a different failure: one dead disk, one failure mode, one local disaster. *

• 3 copies — the original plus two backups, so a single failure never wipes everything.
• 2 different media — e.g. internal disk + external drive/cloud, so one failure mode (a dying disk model, a ransomware-reachable share) can't take all copies.
• 1 off-site — protects against fire, theft, flood at one location.

A crucial extra from the source: disconnect the backup medium after backing up and store it elsewhere. A backup drive left permanently plugged in gets encrypted right along with everything else by ransomware.

Why backup is step 1: it's the only control that lets you fully recover after a disaster — disks die, malware encrypts, people delete. Studies show most people only start backing up after their first loss.

Tip: A backup you've never tested restoring is just a hope. Verify restores periodically.

Go deeper:

• The 3-2-1 backup strategy — Acronis — the origin and rationale of the rule.
• Backup — Wikipedia — backup schemes, media, and restore testing.