External hard drives promise almost unlimited storage: For less than $100, you can add a terabyte of data to your PC or Mac, laptop or desktop. That’s enough for over 750,000 MP3s or photos, or over 230 full-length movie files. Every computer out there, from mega-huge towers to budget-priced Windows tablets, 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 3.5-inch 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. Notebook-class (a.k.a. pocket) hard drives are usually 2.5-inch mechanisms powered through the connector cable . A 2.5-inch model can fit in a coat pocket and some pants pockets.
Desktop-style drives currently top out at 6 terabytes (TB) per mechanism, but some drive manufacturers put two to four mechanisms into a drive chassis for more storage (i.e., two 4TB drives equal 8TB of storage). Notebook-class drives come in capacities up to 2TB, but capacities from 500GB to 1TB are more common.
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 vs. hardware RAID. Read our excellent primer RAID Levels Explained for a more in-depth explanation.
External solid-state drives (SSDs) are found mostly in the notebook-class 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. Besides, unless you’re looking for SSD’s shock-resistance attributes, the drive will be wasted if you use the USB 2.0 interface (rather than, say, Thunderbolt or USB 3.0/3.1) to connect the SSD to your system, since the transfer rate of USB 2.0 is so much slower than either these three interfaces. Thunderbolt, and USB 3.0 external SSD drives are available now, but they are much more expensive than spinning hard drives: for example, a simple 500GB USB 3.0 (spinning) hard drive goes for about $50-60, while a 256GB SSD using USB 3.0 costs from $200 to $400.
Input, Need Input
External drives connect to PCs and Macs via their external cables. USB 2.0/3.0 ports are almost always present; others can include FireWire (400 and 800), eSATA, or more esoteric connectors like USB 3.1/USB-C or iSCSI. Note that while iSCSI uses Ethernet cables, it differs from SAN or NAS technologies, since those connect multiple hard drives to multiple computers. USB 3.1/USB-C and iSCSI are still very rare on drives. iSCSI is mainly used on professional-grade drives like the DroboPro. USB 3.0, becoming the port of choice, provide faster transfer speeds and a minimum of fuss, since almost all desktop and laptop PCs come with USB ports. USB 3.1 is a newer standard, supported using the smaller USB-C connector. It has the same theoretical speed as original Thunderbolt (10Gbps), but is governed by the same group of companies that developed the other formats of USB.
The external drives we’ve reviewed at have at least a USB port, a good thing since even convertible tablets and ultrabooks have at least one USB 2.0 port with its theoretical 480Mbps throughput. Less common, but ostensibly speedier, 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; i.e., you can connect several drives or devices up to a single FireWire port when you connect them together first.
The next fastest interface you’ll see in an external hard drive is the eSATA interface, which is theoretically capable of 3Gbps (3,000Mbps), an order of magnitude faster than USB 2.0. Unfortunately, while eSATA is fast, it does not provide power over the connector cable and will require either a USB cable for power, a combined USB/eSATA cable (and connector) or an external AC adapter. eSATA-compatible drives are on the way out, now that Thunderbolt and USB have replaced eSATA in most applciations. In terms of interface, USB 3.0 is even faster than eSATA, with a 5Gbps theoretical throughput. USB 3.0 has the benefit of being backwards-compatible with USB 2.0 (it will connect to USB 2.0 ports, but will transfer but at the slower USB 2.0 speeds). 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 each additional interface adds to the drive’s complexity and cost. USB 3.1/USB-C uses smaller plugs and jacks compared with traditional USB, but it can use compatible hardware: a handful of USB memory sticks and hard drives are currently available with both USB 3.0 and USB 3.1 support via two separate connectors. Optional adapters will let you use older USB drives with PCs with newer USB-C ports.
Thunderbolt and Lightning: Very, Very Frightening
One of the newer interconnect technologies is Thunderbolt (formerly known as Light Peak), Thunderbolt technology was developed by Intel, and championed by Apple. Thunderbolt was originally designed as a speedy optical link (using fiber optics), but the practicality of adding a new connector to existing systems dictated that the shipping version works with copper cables and existing connectors. Visually (but not electrically) identical to mini-DisplayPort connectors, the Thunderbolt interface can drive both monitors and external hard drives. Like FireWire, Thunderbolt devices can be daisy-chained together to work with one connector on a laptop or desktop. Also like FireWire, the Thunderbolt interface can be used to boot a Mac (USB boot drives may not work on some Macs). Best of all, the Thunderbolt 2 interface has the fastest theoretical throughput: up to 20GBps. Thunderbolt is becoming a common standard in professional-grade systems from makers as diverse as Apple, Dell, and HP. We see Thunderbolt as a niche player, mainly in higher end desktop and mobile workstations, where you need to transfer a lot of data quickly..
A few Dell and HP systems have Thunderbolt ports, but other makers are naturally hesitant to use this more expensive interface. The Thunderbolt drives we’ve reviewed have been promising: The G-Technology G-Dock ev with Thunderbolt and the LaCie 5big have broken our speed records, but all that performance comes at a steep price. These multidrive arrays are best suited to scientists and high-end graphics professionals. AMD and Texas Instruments have announced a competing technology called DockPort or “Lightning Bolt,” but it remains to be seen if much interest here will be in a competing interconnect technology.
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 would be “it depends.” If you are transferring lots of files over a speedy interface like eSATA (fast), USB 3.0/3.1 (faster), or Thunderbolt (fastest), then by all means go for the 7,200rpm 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. USB 2.0 and FireWire 800 are older interfaces that work fine with a 5,400rpm drive. If all-out speed is your goal, multiple drives (7,200rpm, 10,000rpm, or SSD) over Thunderbolt 2 is the fastest (and most costly), with a single SSD connected via Thunderbolt or USB 3.0/3.1 as next fastest, and so on.
After you’ve slogged through the above criteria, you may have to look for other differentiators to find the drive you want. Color 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. If you’re simply using the drive as an extra storage container or if you’re using the backup software built into Windows or Mac OS, bundled software isn’t as important. Warranty is also an big factor in our ratings: Drives can and will fail on you. That cheap drive you found on dealnews.com may only have a one-year warranty. Look for a three- or five-year warranty if you’re hard on your drives.