Why three copies and not one
A single copy protects you from a drive failure, but not from events that hit everything at once: ransomware that encrypts the computer and the connected drive, theft, fire or a deletion that gets synced. The 3-2-1 rule spreads the risk: if one copy fails two remain, and at least one is far away and on a different type of media.
| Copy | Where | Protects against | Example |
|---|---|---|---|
| 1. Original | Your computer or server | — | Internal PC drive |
| 2. Local copy | External drive or NAS | Failures and deletions | NAS with daily backups |
| 3. Off-site copy | Cloud or another location | Theft, fire, ransomware | Cloud service with versioning |
| 3-2-1-1-0 upgrade | Immutable or disconnected copy | Ransomware that deletes backups | Drive unplugged after each backup |
How to read the table
The first two copies are the minimum at home; the third, off-site, is the one almost everyone forgets. The modern 3-2-1-1-0 version adds an immutable or disconnected copy that ransomware cannot delete, and zero errors when restores are verified. For a small business it is the recommended standard.
How to apply it at home
The simplest setup: an external drive with Windows File History or Time Machine on a Mac, and a cloud service for the third copy. Schedule backups so they run on their own; backups that depend on remembering end up not being done.
If you have lots of photos and videos, a two-bay NAS in RAID 1 acts as the local copy for every computer at home, and the NAS itself can send an encrypted copy to the cloud.
How to apply it in a small business
Back up servers and computers to a NAS with daily snapshots, keep at least 30 days of versions and replicate the NAS to the cloud or to another NAS at a different site. Turn on immutability if your NAS or service offers it, so nobody, not even an attacker with your password, can delete recent backups.
Write down who checks the backups and how often. Once a month, restore any file to confirm everything works.
How much space you need
A full copy takes as much space as your data, and daily versions only add what changes. With 1 TB of data and 5% daily change, 30 days of versions take about 2.45 TB; the backup storage calculator gives you the figure for your own data.
Choose a drive or NAS with at least twice the capacity of your current data to leave room to grow.
What to back up and what not
Prioritise what you cannot get back from anywhere else: documents, photos, family videos, accounts, databases and your software settings. The operating system and applications can be reinstalled; your files cannot.
In a business also include email if no external provider keeps it, virtual machines and exported configuration of routers, firewalls and NAS units. Without it, recovering from a failure can take days.
How to check your backups work
A backup you have never restored is a backup you do not know exists. Every month try to recover a specific file and, at least once a year, simulate losing a whole computer: how long it takes, what is missing and which passwords you need.
Keep the encryption passwords for your backups somewhere other than the data itself, for example in a password manager with its own backup or printed and stored somewhere safe.
Common mistakes
Confusing sync with backup: if you delete a file in a synced folder, it is deleted in the cloud too. Confusing RAID with backup: it protects against failures, not deletions. And never testing a restore: many businesses find out their backups did not work on the very day they need them.
Frequently asked questions
Does an always-connected external drive count as a backup?
It counts as a local copy, but ransomware can encrypt it just like the computer. Unplug it when the backup finishes or use a NAS with snapshots that cannot be modified.
How often should I back up?
As often as your data changes: daily for work documents and weekly for personal photos. Ask yourself how many days of work you can afford to lose.
Is the cloud enough as the only backup?
No: if you delete something by mistake or your account gets locked, you can lose access. Combine it with a local copy that you control.
Should I encrypt the backups I upload to the cloud?
Yes, especially if they contain personal or customer data. Most backup programs and NAS units can encrypt before uploading, so the provider cannot read the contents. Keep the key safe and somewhere separate: without it nobody can restore anything, not even you, and the provider will not be able to help.