The short answer
Three copies of anything you care about, on two different kinds of storage, with one of them somewhere else. A single external drive sitting next to the computer is not a backup against fire, flood or theft.
- Copy one: the working file on the machine.
- Copy two: an external drive, backed up automatically rather than when you remember. File History on Windows, Time Machine on a Mac.
- Copy three: offsite. Cloud storage, or a second drive kept at another address.
- Test a restore. An untested backup is a hope, not a backup, and the commonest failure is discovering it has silently not run for months.
- Photographs need this most, because they are the thing nobody can replace.
What will ruin it
Cloud sync is not a backup. If you delete a file or ransomware encrypts it, the sync dutifully propagates that to every device. Real backup keeps versions you can go back to.
Why it works
Backup protects against different failures: drive failure, accidental deletion, theft, fire and ransomware. One copy in one place defends against only the first, which is the least likely of the five.
Tipking is general household information, not professional advice. Test anything on a hidden patch first. Full disclaimer.
More computers
Why your computer is slow, and what actually fixes itHow to tell a phishing email from a real oneHow to make passwords you do not have to rememberWhat to do before you get rid of an old computerHow to stop a laptop overheatingAppliance Running Cost CalculatorWhat any appliance costs per use, per week and per year, plus eighteen worked out for you at today's rate.Open the calculator →