What is the 3-2-1 backup rule?
The short answer
Keep 3 copies of your data on 2 different types of storage, with 1 copy off-site. In practice that is the original on your computer, a local copy on an external drive or NAS and a cloud backup. The rule has lasted because one setup covers a dead drive and theft as well as fire and ransomware.
In detail
The 3-2-1 rule comes from photographer Peter Krogh. Security agencies including CISA and US-CERT adopted it because it matches how data really gets lost. A single drive has a real chance of failing in any given year. Backblaze’s well-known Hard Drive Stats show that year after year. Two copies in the same house can still be lost together in a fire, flood or burglary. The third copy sits off-site. That is the copy that gets you through a fire or a break-in.
Setting it up today costs less than people expect. Copy one is your computer as it is. Copy two is an external SSD or NAS that backs up automatically. Time Machine does that on a Mac and File History on Windows. Acronis adds full disk images. Copy three is a cloud backup service that runs quietly in the background. Backblaze costs around $99 a year for unlimited data. IDrive covers 5 TB across all your devices. All in, you pay for one drive plus under $10 a month.
Professionals add two habits. The first is versioning. The off-site copy should keep file history for 30 days or more. Without versions, files encrypted by ransomware just become three copies of unreadable files. The second is a restore test now and then. Open the backup app twice a year and restore a folder. Most backup problems are human rather than technical. A backup set up once can quietly stop working for months, and a test shows you in time.