Why the boring specifications matter more than the case

Walk into any electronics shop and the external drives are arranged like sweets: brushed aluminium, glossy plastic, a little light that glows when you plug it in. It is tempting to choose on looks. Please don't. A backup drive has one job — to hold a second copy of everything you would hate to lose — and the things that decide whether it does that job well have nothing to do with how it looks on a shelf. Capacity, speed and reliability are the three that matter, and all of them are printed in small type on the side of the box.

What follows is a practical guide for ordinary home users: what to look for, what to ignore, and how to set the drive up so that backups actually happen rather than being a thing you mean to do one day.

Work out the capacity you need, then double it

Start by estimating how much data you currently have. On a laptop, right-click your main folders and check the total size; most household machines hold somewhere between 150GB and 600GB of documents, photos, music and downloads. If you have a large photo library or you edit video, the figure can easily run into several terabytes.

The rule of thumb is to buy at least twice the space you are protecting. There are two reasons. First, nobody's data gets smaller. Second, and more importantly, proper backup software keeps versions — older copies of files you have since changed or deleted. That is the whole point of a backup: if you accidentally overwrite a spreadsheet, you want last Tuesday's copy as well as today's.

• Under 500GB to protect: a 1TB drive gives comfortable room.
• 500GB to 1TB to protect: buy 2TB, ideally 4TB.
• Photo or video libraries: a 4TB or 8TB desktop drive, and plan for more.

Portable or desktop: which shape suits you?

Portable drives are the small ones that run off the USB port alone. They usually house a 2.5-inch mechanism spinning at around 5,400rpm, which gives you roughly 100 to 130MB per second, and they top out at about 5TB. They fit in a coat pocket, need no power socket and are ideal for a laptop owner who wants to run a backup after work and unplug.

Desktop drives are the chunkier boxes with a separate power supply. They hold 3.5-inch mechanisms, usually spin faster, offer far more capacity for the money and often include better cooling and shock protection. The trade-off is that they live in one place. That is not always a disadvantage.

• Choose portable if you have under 2TB to protect, or you want to keep the drive in a drawer or take it with you.
• Choose desktop if you have a large library, want 4TB or more, or you intend to leave the drive permanently connected.
• If you leave a drive permanently plugged in, remember that a power surge or a nasty piece of ransomware can reach it too. Unplugging it between backups is stronger protection than any specification.

Speed: how much do you actually need?

Speed only matters at two moments — the first full backup, and the day you have to restore everything after a failure. For a routine that runs overnight while you sleep, a standard spinning drive is perfectly adequate. An external solid-state drive is silent, has no moving parts and will finish a backup many times faster, but it costs several times more per gigabyte. For most homes it is an unnecessary luxury for backup, though it makes an excellent working drive for video editing.

Whatever you buy, insist on a USB 3 connection of some kind. A drive advertised as USB 3.0, 3.1 or 3.2 is fine; anything USB 2.0 only will crawl. Check the port on your computer before you buy, and use the cable supplied with the drive. If it is a small bus-powered model, plug it directly into the machine rather than into an unpowered hub, which can cause the drive to drop out mid-backup.

Reliability: the part nobody advertises loudly

Every drive fails eventually, so buy one from an established manufacturer with a proper warranty — two years is normal, three is better — and keep the receipt in a safe place. A few details are worth looking for.

• Shock sensors and ramp parking on portable models, which park the heads safely if the drive is knocked.
• Health reporting, so free drive-monitoring utilities can warn you before a failure rather than after.
• Decent ventilation. Heat is the enemy of long life; a vented desktop enclosure beats a sealed plastic brick.
• Avoid unbranded caddies with unknown drives inside and no warranty. They are cheap for a reason.

Treat the drive kindly. Do not move a spinning drive while it is working, and give it a flat, stable surface rather than the arm of the sofa.

Build a routine the drive can keep

The best drive in the world is useless if it sits in a drawer. Set it up the day it arrives, using the backup tool built into your operating system or a reputable third-party package. Schedule it to run daily at a time when the computer is switched on but you are not using it — overnight is ideal.

• Use versioned backups, not a simple mirror. A mirror copies your mistakes and deletions straight across.
• Follow the old 3-2-1 rule: three copies of anything important, on two different types of media, with one kept off-site — at a relative's house, for instance, or in cloud storage.
• Test a restore once or twice a year. A backup you have never restored from is a hope, not a backup.

Finally, a short checklist before you part with your money: know how much you are protecting and double it; pick portable for convenience, desktop for capacity; insist on USB 3 and a known brand with a warranty; skip the solid-state drive unless speed genuinely matters; and budget for a second copy somewhere else. Do that, and you will have a backup setup that quietly does its job for years.