What is the 3-2-1 backup rule?

The 3-2-1 rule is the long-standing shorthand for backup coverage that survives realistic disasters:

• 3 copies of the data - the live one plus two backups.
• 2 different media or systems - so one failure mode cannot take both.
• 1 copy off-site - because fire, flood and theft take everything in the building.

Cloud storage can serve as the off-site copy, but only if it is a genuine backup rather than a synchronised folder.