You can never have too much digital storage, and the day will come—sooner than you think—when you won’t be able to squeeze a single new file onto your computer’s hard drive. And if your primary computer is a laptop or an all-in-one desktop, you won’t be able to solve the problem by opening the case and tossing in a supplemental drive. 

One solution might be to rent storage space in the cloud, but buying a hard-drive’s worth of capacity is prohibitively expensive: 500GB of storage on Dropbox, for example, will set you back $499 per year. If you need just storage, as opposed to a service for file syncing or collaborating via the cloud, buying a portable hard drive is far more economical. For around £100, you can get a 2TB drive that supplies four times the capacity of a Dropbox account. Pay for that storage capacity once, and you’ll own it forever—and you can take it with you wherever you go. Before you can choose the right drive, however, you have to identify your needs, wants, and budget.

Mac or PC: OS X and Windows use different file systems (HFS+ and NTFS, respectively), so most hard-drive manufacturers offer platform-specific models; the drives are pre-formatted accordingly, and the bundled software (if any) is compatible with the given platform. OS X can read files on an NTFS drive, but it can’t write them. If you intend to use the same drive on both platforms, you can install software on your Mac that will enable it to do both: NTFS-3G is a free option. If you prefer commercial software, take a look at Paragon NTFS or Tuxera NTFS.

Capacity: To determine how much storage you need, consider adopting this rule of thumb from drive manufacturer Western Digital: A 500GB hard drive can store approximately 100,000 digital photos taken with a 6-megapixel camera, or 125,000 songs encoded as 128-kbps MP3 files. Higher-resolution photos and music, of course, consume more storage. 

Seagate

Everything else being equal, a high-capacity drive will deliver a better price-to-performance ratio than a low-capacity model. You won’t regret buying more storage capacity than you currently need, because you will surely need more later.

Rotational speed: The rate at which a hard drive spins its platters has a direct effect on how fast it can read and write data. A drive spinning at 7200 rpm will deliver much better performance than a drive spinning at 5400 rpm. Some high-end desktop drives spin their platters at 10,000 rpm.

Drive interface: Once you’ve decided on the speed and capacity you need, you need to consider how the drive will connect to your computer. USB is the most common interface for Macs and PCs, and USB 3.0 delivers a faster data rate than USB 2.0 (5 gbps versus 480 mbps) and more electrical power to an attached device (900mA versus 500mA). The newer standard is backward-compatible, so your computer will be able to use a USB 3.0 drive even if the computer has only USB 2.0 ports.

Thunderbolt ports are twice as fast as USB 3.0 ports, achieving a raw data transfer rate of 10 gbps. That’s speedy enough to transfer a full-length, high-definition movie in less than 30 seconds. Apple provides Thunderbolt ports on its most recent desktop and laptop computers, and the technology is beginning to show up on Windows machines, too. Thunderbolt hard drives are relatively expensive, however: At £150, Buffalo’s 500GB MiniStation HD-PA500TU3 portable drive costs nearly as much as a 2TB hard drive equipped with a USB 3.0 interface. Still, If you choose to buy one of Buffalo’s drives, you’ll be happy to know that the company includes a Thunderbolt cable in the box, given ho pricey these cables are: Apple’s 2-meter cable costs £39.

FireWire (also known as IEEE 1394) is another high-speed interface used on both Macs and many PCs. The FireWire 400 interface can support a data transfer rate of 400 mbps, while the newer FireWire 800 interface can deliver throughput of 786 mbps.

USB, Thunderbolt, and FireWire all provide enough electrical power to run an attached drive, so the only cord you’ll need to carry with you is the appropriate interface cable.

Enclosure: The vast majority of portable hard drives are 2.5-inch mechanisms, but not all portable hard drives are the same size. Some models come housed in low-profile enclosures, while others are wrapped in shock-absorbing material within ruggedised cases. Such design decisions affect the drive’s overall weight, but they also influence how well the drive can survive misadventure. If you’re a frequent traveler who grudges every ounce that goes into your laptop bag, you’ll need to work out for yourself the right balance between data security and tolerable shoulder load.

Some manufacturers, including Seagate and Western Digital, offer accessory cases for their drives that can add shock protection. We especially like the Nomad hard-shell case for Western Digital’s Passport drives. The amply padded, 6.25-ounce polycarbonate case has an opening for a USB cable, so you don’t have to remove the drive to use it. 

Other features: In situations where performance is roughly equal, the easiest way for a hard drive manufacturer to differentiate its product from the competition's is by adding special features or by bundling the drive with useful software. If you’re interested in backing up your PCs, watch for drives that come bundled with automated backup software. 

If you’re security conscious, look for a drive that you can password-protect or that includes software for encrypting the data stored on it. Some Seagate drives support the Universal Storage Module standard: They use SATA as their primary hardware interface, but you can switch to a USB, Thunderbolt, or FireWire interface by plugging in the appropriate module. Seagate is also unique in offering a portable drive that can wirelessly stream media to a mobile client device.

The best portable hard drives

LaCie Porsche P'9223 Slim

We said: If you need an affordable, high-capacity storage device then a conventional hard disk is still the best option. But if you’ve got a MacBook that has an internal SSD drive and you want an external back-up drive that provides equally high performance then the Porsche P’9223 Slim is one of the best and most affordable SSD drives currently available.

Read our LaCie Porsche P'9223 Slim review

Verbatim External SSD

While it may not be the fastest solid-state drive currently available, the External SSD is definitely one of the cheapest and is well worth considering if you just want a compact and affordable back-up device for your laptop.

Read our Verbatim External SSD review

LaCie Rugged USB 3 

Some people might prefer the low-cost, pocket-sized drives from Kingston and Verbatim for sheer convenience and portability. However, those two drives are also quite a bit slower than the Rugged drive and lack the versatility of its twin interfaces. So if you want a sturdy SSD drive that is fast, affordable and can also be used with older non-Thunderbolt machines then the Rugged drive is hard to beat. We gave it 5 stars. 

Read our LaCie Rugged USB 3 review

Elgato Thunderbolt SSD review

The Thunderbolt SSD does work well when handling larger file sizes, and its sturdy design will appeal to photographers or videographers working on outdoor shoots. However, its more modest performance with smaller file sizes suggests that it’s not the best choice for general home or office use. And, at this price, it should also include additional features such as the alternative USB 3.0 interface found on the less-expensive Rugged drive from LaCie.

Read the Elgato Thunderbolt SSD review

Buffalo MiniStation Thunderbolt

The MiniStation Thunderbolt performs well, but like all Thunderbolt drives it is very expensive and many people may prefer to opt for a more affordable USB 3.0 drive instead. However, it’s a good option for people who require a fast, portable drive that they can carry around with their laptop

Read the Buffalo MiniStation Thunderbolt review

Seagate Slim Portable

The MiniStation Thunderbolt performs well, but like all Thunderbolt drives it is very expensive and many people may prefer to opt for a more affordable USB 3.0 drive instead. However, it’s a good option for people who require a fast, portable drive that they can carry around with their laptop. 

Read our Seagate Slim Portable review

Iomega SSD Flash

Flash drives are still very expensive, as well as being relatively limited in capacity, so people on a tight budget may well prefer to stick with a conventional hard drive for the time being. However, the SSD Flash is still a good option for MacBook Air owners who want a really lightweight drive that they can carry around without piling on the pounds.

Read our Iomega SSD Flash review

The best ultra portable hard drives and USB sticks

Kingston Data Traveller Hyper X 3.0

It may not be the fastest storage device in this group, but the Hyper X 3.0 is much faster than a USB 2.0 drive, and is a good option for people who simply want a light, portable backup drive that you can carry around in your pocket. 

Read our Kingston Data Traveller Hyper X 3.0 review

LaCie Rugged Key

The Rugged Key is a lot more expensive than an ordinary memory stick, but it could be a good investment for people who regularly work outdoors with their laptop and need extra protection for their important work files. Its encryption option also provides another layer of security in case the Rugged Key gets lost or stolen.

Read our LaCie Rugged Key review

Best of the rest

Buffalo MiniStation Thunderbolt

Buffalo MiniStation Thunderbolt: The MiniStation sports both a USB 3.0 and a Thunderbolt interface, but the drive’s rather pedestrian rotational speed of 5400 rpm hobbles its overall performance. It comes pre-formatted for the Mac, and both USB 3.0 and Thunderbolt cables are included.

Rocstor Lancer LX

Rocstor Lancer LX: If you’re looking for a ruggedised drive with a very fast interface, Rocstor’s Lancer LX provides both USB 3.0 and two FireWire 800 ports. The border of the enclosure is fabricated from aluminium and can bear a fair amount of weight, while shock-absorbing material inside the case protects the drive from thumps and bumps. 

Seagate Satellite

Seagate Satellite: This battery-powered drive can create its own local hotspot, and stream audio and video to up to eight wireless clients. Seagate provides client software for Android and iOS devices, plus an NTFS driver for the Mac. It uses a USB 3.0 interface.

Seagate Slim

Seagate Slim: The 9mm-thin Seagate Slim is appropriately named. It leaves the factory with a USB 3.0 interface, but since it’s based on the Universal Storage Module standard, you can purchase an optional Thunderbolt interface (you’ll need to provide your own cable). It comes formatted for NTSF, but Seagate provides an NTFS driver for the Mac. On the downside, this model delivers only 500GB of capacity. 

Western Digital My Passport

Western Digital My Passport: For people who need a lot of storage on the go, this drive packs up to 2TB of space into a package that weighs just 8 ounces. It comes with a USB 3.0 interface and a collection of utilities—including automatic back-up software—for both the Mac and the PC.