A rule of thumb that has survived four decades of storage change deserves attention, but repeating it without understanding what it defends against is how organizations end up compliant on paper and unrecoverable in practice.
What the Rule Says
Three copies of the data, on two different media types, with one copy offsite. Each clause defends against a distinct failure: corruption, media-class failure, and site loss respectively.
Why Each Clause Exists
Multiple copies survive corruption of one. Different media survive a defect common to a media class. The offsite copy survives fire, flood, or anything else that takes the building. Drop a clause and you drop its specific protection.
The 2026 Condition
The original 3-2-1 backup formulation predates ransomware that hunts backup catalogs, which is why modern variants add immutability and verification to the original three clauses.
Compliance Versus Protection
Three copies an intruder with production credentials can reach are, functionally, one copy. Counting copies satisfies the rule; making at least one of them unreachable is what satisfies the threat.
Applying It Honestly
Map your actual copies against each clause and ask what failure each one survives. Most estates discover a clause they satisfy nominally rather than genuinely, and that gap is the useful output of the exercise.
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