A guide to buying an external hard drive

External hard drives can deliver large amounts of cheap extra storage – these days, you can add 1TB of space to your PC, Mac or laptop for just £50. That's enough for over 750,000 MP3s or photos, or over 230 DVD-sized movies. Every computer out there, from mega-huge towers to slim Ultrabooks, can connect to at least one hard drive. If you're lucky enough to have multiple input/output ports, you can hook up many more. Auxiliary storage allows you to back up your system files, in case your primary system goes kaput.

Hard drive types

There are two types of external drives. Desktop-style drives, with 3.5in mechanisms inside, require a power adapter. Desktop drives are designed to stay in one place, usually on your work surface at home or at the office. If you're buying a desktop-style drive for active use (video or lots of file transfers), look for one with a built-in fan, as the extra cooling will extend the drive's life expectancy. More compact notebook-class hard drives are usually 2.5in disks powered through the connector cable without the need for a power adapter. These are portable and can fit in your coat pocket, or perhaps even your trouser pocket.

Desktop-style drives currently top out at 4 Terabytes (TB) per mechanism, but some drive makers put two to four mechanisms into a drive chassis for more storage (i.e. two 4TB drives equal 8TB of storage). Notebook-style drives come in smaller capacities in the 500GB to 2TB range.

A word about multiple drives: You can increase capacity, speed or data protection by buying an external RAID array, but multiple drives add expense and (some) complexity. Once you connect a simple (single volume) external RAID array to your PC or Mac, it will show up and act as any other external drive. After that, it can become more complex. You should consider a drive with support for RAID levels 1, 5, or 10 if you're storing really important data that you can't afford to lose. There are other RAID levels for speed, capacity, and other factors like software versus hardware RAID. Please read my colleague Samara Lynn's excellent primer on RAID levels for further details.

External solid-state drives (SSDs) are found mostly in the notebook-style form factor, but these are still relatively rare because they're pricey in terms of cost per gigabyte. They're currently limited to smaller capacities, specifically in the 64GB to 512GB range. We recommend that you buy SSDs for use as internal rather than external drives, because if you want a half decent capacity – which let’s face it, you need for a backup disk – you’re going to be paying a heck of a lot of money.

Input, need input

External drives connect to PCs and Macs via their external connectors, which can be a USB 2.0 port, or hopefully these days USB 3.0 which offers much faster transfer speeds. Other inputs can include FireWire (400 and 800), eSATA, or more esoteric connectors like iSCSI. Note that while iSCSI uses Ethernet cables, it differs from SAN or NAS technologies, since those connect multiple hard drives to multiple computers. iSCSI is still very rare on drives, and mainly used on professional-grade drives like the Drobo Pro.

The external drives I look at have at least a USB port, and hopefully it’s of the USB 3.0 variety. This transfers files much more quickly – for example, in previous tests in our labs, we’ve seen a 1.22GB folder shifted across in 19 seconds via USB 3.0. Of course, the machine on the other end of the drive will also need a USB 3.0 port to reach these faster speeds. USB 3.0 will work fine with USB 2.0, by the way – it’s backwards compatible – but you’ll get USB 2.0, not 3.0, speeds.

Less common is the FireWire port, in both 400Mbps and 800Mbps formats. FireWire 400 and 800 are signal-compatible (they can use the same wires), but they have different FW400 or FW800 connectors on the ends of those cables. FireWire can be daisy-chained; in other words, you can connect several drives or devices up to a single FireWire port by linking them all up to each other in a chain.

Then there’s also eSATA, which is faster than USB 2.0, but not as swift as USB 3.0 – and it doesn’t provide power over the connector cable. You can find drives with multiple ports (for example a triple interface drive with USB 2.0/3.0, FireWire 800, and eSATA), though you'll still only be able to connect a single drive to a single computer, and bear in mind that each additional interface adds to the drive's complexity and cost.

Is drive speed important?

Some drive manufacturers will crow about the speed of their drive mechanisms. While a 7,200rpm drive is inherently faster than a 5,400rpm drive, the true answer to the question of whether drive speed is really important would be “it depends.”

If you are transferring lots of files over a speedy interface like eSATA (fast), USB 3.0 (faster), or Thunderbolt (fastest), then by all means go for the 7,200 rpm drive. However, if you're limited to USB 2.0 or FireWire 400/800, then I would trade speed for capacity and get the largest 5,400rpm drive that your budget allows – those older interfaces work fine with a 5,400rpm drive. If all-out speed is your goal, multiple drives (7,200rpm disk, 10,000rpm disk, or SSD) over Thunderbolt is the fastest (and most costly, as we’ve already mentioned), with a single SSD connected via Thunderbolt or USB 3.0 as next fastest, and so on.

Other considerations

After you've slogged through the above criteria, you may have to look for other differentiators to find the drive you want. Colour and design are usually a concern: A drive you're embarrassed to use won't be used at all, defeating its purpose. Included software is a concern if you don't already have a backup plan, as well. Indeed, if you want to automate backups to your external drive, you’d do well to consider a solution such as Seagate’s Backup Plus.

Warranty is also an important factor – hard drives can and will fail on you. If you’re intending to use your external drive extensively, it could be worth paying a bit more and looking for drives with a three or five year warranty. There’s a reason these drives are guaranteed for more than the bog-standard one year warranty.