3-2-1 backup rule: 3 copies of critical data, on 2 different media types, with 1 stored offsite — redundancy protects against correlated failures

Why This Is a Rule

Data loss events are correlated: a laptop theft loses both the laptop and the external drive in the same bag. A house fire destroys both the computer and the local backup drive. A cloud service outage affects both your primary and backup accounts on the same platform. A ransomware attack encrypts both the local drive and the connected backup. Each of these scenarios defeats backup strategies that don't account for correlation.

The 3-2-1 rule defeats correlated failures through diversification across independent failure domains. 3 copies ensures that even if two copies are lost simultaneously (unlikely but possible), one survives. 2 different media types (e.g., SSD + cloud, or local drive + external drive) ensures that a media-type-specific failure (drive firmware bug, cloud service shutdown) doesn't affect all copies. 1 offsite ensures that a location-specific disaster (fire, flood, theft) doesn't affect all copies.

For knowledge workers, "critical data" means your notes, writing, projects, financial records, and any data that can't be recreated. The years of accumulated thinking in your PKM system, the archive of completed work, the financial records — losing these is not just inconvenient but potentially devastating. The 3-2-1 rule makes catastrophic data loss statistically near-impossible.

When This Fires

• When setting up backup infrastructure for your critical data
• When reviewing existing backup strategies for adequacy
• When any single point of failure exists in your data protection
• Complements Choose open portable formats (Markdown, CSV, JSON) over proprietary — migration cost compounds daily with every new piece of locked-in data (open portable formats) with the backup infrastructure that protects the data regardless of format

Common Failure Mode

Single-backup complacency: "I back up to an external drive, so I'm protected." A single backup on the same desk as your computer is defeated by theft, fire, flood, or power surge. You have 2 copies on 1 media type with 0 offsite — failing on two of the three criteria.

The Protocol

(1) Identify your critical data: notes, writing, projects, financial records, photos, code. (2) Implement 3 copies: Copy 1 — primary working copy (laptop/desktop). Copy 2 — local backup (external drive, NAS, or separate internal drive). Copy 3 — offsite backup (cloud service, remote server, or physical drive stored elsewhere). (3) Verify 2 different media types: if copies 1 and 2 are both on SSDs, make copy 3 on a different medium (cloud storage, magnetic drive). (4) Verify 1 offsite: at least one copy must be in a different physical location. Cloud backup satisfies this automatically. (5) Automate: backups that require manual action will eventually be skipped. Set up automated backup for copies 2 and 3 so protection doesn't depend on remembering.