The 3-2-1 backup rule (and why RAID isn't a backup)
The most quoted rule in data protection, in one line: 3 copies of your data, on 2 different kinds of media, with 1 copy off-site.
What each number means
- 3 copies. Your live data counts as copy one. You want two more, so that any single loss still leaves you with a working version.
- 2 media. Keep the copies on separate hardware — not two folders on the same drive, and ideally not two drives behind the same controller. Different failure domains mean one fault can't take both.
- 1 off-site. One copy lives somewhere else — a friend's house, a safe-deposit box, or cloud storage — so fire, theft, or flood can't destroy every copy at once.
Why RAID is not a backup
A RAID array protects against a drive dying. It does nothing about the threats that actually lose most people's data: accidental deletion, a botched overwrite, ransomware, a corrupt filesystem, a power surge, or a controller that scrambles every disk. All of those write straight through to every copy in the array.
RAID is uptime, not safety. It keeps a server running through a disk failure. It is one copy of your data, however many drives it spans — so in 3-2-1 terms a RAID NAS is still just copy one.
A practical home setup
- Copy 1: your NAS or main PC (where you work).
- Copy 2: an external or second internal drive that you back up to on a schedule — different hardware, same building.
- Copy 3: a drive you rotate off-site, or an encrypted cloud copy.
How many drives do you actually need?
Size it for the data you have plus a few years of growth, with free space to spare. Our 3-2-1 backup planner turns your data size and growth rate into a drive count per copy — then find the cheapest drives by price per TB.
General guidance, not a guarantee. Test your restores periodically — a backup you've never restored from is only a hope, not a copy.