3-2-1 backup rule
The rule of keeping at least three copies of data on two different types of media, with one copy stored elsewhere, separate from the main systems.
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Why it matters
Backup is the last safeguard against very different events: hardware failure, accidental deletion, ransomware and natural disasters. A backup on the same server or network can be lost together with the system. The modern form of the rule also recommends one copy that is immutable or offline, and regular restore tests.
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Example
A company keeps its data on the server, on a separate backup device in the office and encrypted in cloud storage. When ransomware encrypts the server and the office device, the company recovers from the cloud copy within two days.
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Common mistake
Feeling safe because backups exist without ever testing a restore. A broken or incomplete backup is only discovered when you need it; restores should be tested at regular intervals.
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