A Plain-English Guide to Backing Up Your Data (and the 3-2-1 Rule)

That’s the situation this post is about. Not “what if you don’t back up,” but “what if the backup you already have quietly isn’t as good as you think it is.”

What backup is actually protecting against

People tend to talk about “backup” like it’s one thing, but it’s really covering a few different ways things can go wrong:

• Accidental deletion. You (or someone else with access) removes a file you needed.
• Hardware failure. A drive dies, a controller fails, a laptop gets dropped.
• Theft, fire, or flood. The whole device is gone, not just the data on it.
• Ransomware. Something actively encrypts or deletes your files, and reaches for your backups too if it can.
• Quiet data corruption. The file is still sitting there, but its contents have changed or degraded without anyone noticing.

A single backup copy on a second drive covers the first two reasonably well. It doesn’t help much with theft or fire if that drive sits next to the computer it’s backing up, and it does nothing at all for corruption that happened before the copy was made. That’s the gap the 3-2-1 rule is built around.

The 3-2-1 rule, plainly

The rule is usually credited to photographer Peter Krogh, who wrote about it in the early 2000s while working out how to manage huge digital photo archives. Photographers are a good audience for this advice: their files are often irreplaceable and there’s rarely an IT department involved. The rule itself is three numbers:

• 3: keep three total copies of your data, the original plus two backups.
• 2: put those copies on at least two different types of media or device (not two folders on the same drive).
• 1: keep at least one copy offsite, somewhere other than where the original lives.

A simple version of this in practice: your working files on your laptop, a second copy on an external hard drive, and a third copy in a cloud storage account. Three copies, two media types (local disk and cloud), and one of them offsite.

Why each number is there

None of the three numbers is picked at random.

Three copies exist because backups fail too. A backup drive can die on its own, get overwritten by accident, or sit there quietly failing for months before anyone checks on it.

Two media types matter because whole categories of storage can fail together. A bad batch of a particular drive model, a flaky USB controller, a format that stops being readable after an OS update: any of these can take out the original and its backup in the same event if they’re on matching hardware.

One offsite copy exists because some failures have nothing to do with the storage medium itself. A fire, a flood, or a burglary takes out everything in the room, backup drives included.

The newer version: 3-2-1-1-0

The original rule comes from a time before ransomware was a mainstream problem and before cloud storage was cheap or common. It was built around tape and physical media. Ransomware changes the picture: it doesn’t just lock up your working files, it goes looking for connected backup drives and cloud-synced folders to encrypt or wipe out too, specifically so you can’t recover without paying up. A backup that stays connected and writable all the time is one ransomware can reach.

That’s led people to extend the rule into what’s often written as 3-2-1-1-0:

The extra “+1” is an offline or immutable copy: a backup that’s either physically disconnected most of the time (an external drive you plug in only to run a backup, then unplug again) or stored somewhere it can’t be changed or deleted for a set period, even by someone with admin access.

The “+0” means zero errors: backups actually get tested and restored occasionally, not just created and left alone. A backup nobody has ever restored from is really just a guess.

A simple setup for one person

None of this requires enterprise software. For someone with a photo library, a set of project files, or a personal archive, a reasonable starting point looks like this:

1. Working copy: the files on your main computer.
2. Local backup: an external drive, ideally one you plug in to back up and then disconnect, rather than one that stays plugged in permanently.
3. Offsite backup: a cloud storage account, or a second drive kept somewhere else entirely (a relative’s house, a safe deposit box).

That covers 3-2-1 on its own. Disconnecting the external drive between backups also gets you most of the way to the “+1.”

The part none of this covers

3-2-1, and 3-2-1-1-0 along with it, is really about how many copies you keep and where you keep them. Neither version says anything about whether the data inside those copies is still intact.

Go back to the wedding photographer. If a file had already picked up a few bad bits before it was ever backed up, copying it to an external drive and a cloud account doesn’t undo that. It just puts three copies of the same broken file in three places, which technically satisfies the rule while protecting nothing at all. The “0 errors” idea gets closest to catching this, but “verified” in most setups just means the restore job completed, not that the file’s bytes match what they were the day it was created.

That’s a genuine hole in an otherwise solid rule, and it’s worth its own post, specifically what it actually takes to notice corruption at the file level instead of just at the “backup job finished” level.

Recap

• 3 copies of your data
• 2 different media types
• 1 copy offsite
• 1 copy offline or immutable
• 0 errors: verify your backups actually restore, not just that they ran

Sources for the claims above are listed below.