3-2-1 Backup Rule
The guideline of keeping three copies of data, on two different media types, with one copy off-site.
Formula / rule
3 copies, 2 media, 1 off-site
Measured in
count
In depth
Three copies protects against individual drive failure, two media types against a systematic flaw in one technology, and the off-site copy against fire, flood and theft — the scenarios that destroy every local copy simultaneously. Cloud storage satisfies the off-site requirement conveniently, provided the upload has actually completed.
Examples in practice
Working drive, local backup drive and encrypted cloud copy together satisfy the rule for an entire archive.