How big an external drive do I need for backups?

People size a backup drive against the capacity of their computer, which is the wrong number twice over — the machine is rarely full, and the backup needs room for history rather than a single copy.

Step by step

1. Measure what you are actually backing up.Look at the used space in your user folder rather than the whole drive. The operating system and applications are only included if you are imaging, and they compress well.
2. Multiply by two to three for versioning.Versioned backups keep old copies of changed files and deleted files for a retention period. That history is the feature that saves you from your own mistakes, and it needs space to live in. Two times is workable, three is comfortable.
3. Add for growth.Estimate what you added over the last year — usually photographs and video — and allow for several more years of that. Replacing a backup drive early is a nuisance you can avoid at purchase for a few pounds.
4. Decide whether the image is included.If you are also keeping disk images, add the size of the used portion of the system drive per image generation you intend to keep. This is often what pushes a household from 1 TB to 2 TB.
5. Note the difference between advertised and usable capacity.Drives are sold in decimal terabytes and reported by the computer in binary, so a 1 TB drive shows as about 931 GB. Nothing is wrong and nothing is missing; it is two ways of counting. Build it into your sums.
6. Buy two if the data matters.Two smaller drives on rotation, one kept elsewhere, beats one large drive for almost every household. The offsite copy is the leg most people are missing.
7. Choose the type honestly.A mechanical drive gives far more capacity per pound and is fine for backups, which are sequential and unattended. An SSD is faster, more robust in a bag, and costs several times as much per terabyte. For a drive that sits in a drawer, mechanical is the sensible buy.

Tips

• A portable drive powered over its cable is easier to keep offsite than a desktop drive with a mains adapter.
• Do not buy a drive so large that you never fill it and never replace it. Drives age; a backup drive over about five years old should be replaced whatever its health readings say.
• If you are protecting several machines, size for the combined total plus versioning, or give each machine its own drive to avoid one failure taking out every backup at once.
• Label the drive with what it holds and the date it was last verified. Unlabelled drives in a drawer become drives nobody dares erase.

Questions people ask