External hard drives promise almost unlimited storage: For under $100, you can add a terabyte of data to your PC or Mac, portable or desktop. 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 compact nettops and netbooks, 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 (aka pocket) hard drives, like the Apricorn Aegis Padlock Pro ($759.00 direct, 4.5 stars) and Iomega eGo ($199.99 list, 4 stars), are usually 2.5-inch or 1.8-inch mechanisms powered through the connector cable without the need for a power adapter. A 2.5-inch pocket drive can fit in a coat pocket and some pants pockets, while 1.8-inch drives can easily fit in your jeans.
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 capacities up to 1.5TB, but capacities from 250GB to 750GB 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. Please read Samara Lynn's excellent primer RAID Levels Explained for more.
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. 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, eSATA, Thunderbolt, or USB 3.0) to connect the SSD to your system, since the transfer rate USB 2.0 is so much slower than either these three interfaces. eSATA, 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 2.0 (spinning) hard drive goes for about $80, A 240GB dual SSD using Thunderbolt is over 10X that at $899.99.
Input, Need Input
External drives connect to PCs and Macs via their external connectors. USB 2.0 ports are almost always present; others can include FireWire (400 and 800), eSATA, or more esoteric connectors like Wireless USB, USB 3.0, 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. Wireless USB and iSCSI are still very rare on drives. Wireless USB drives are now mostly a curiosity since NAS drives are easier to manage, and iSCSI is mainly used on professional-grade drives like the Drobo Pro. USB 3.0, becoming the port of choice, provide faster transfer speeds like in the HP Portable External Hard Drive (1TB) ($169.99 direct, 4 stars), which was able to transfer a 1.22GB folder in about 19 seconds in our labs.
The external drives I look at have at least a USB port, a good thing since even netbooks and ultralight notebooks 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 also currently the most expensive. 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.
Thunderbolt and Lightning, Very Very Frightening
The newest interconnect technology 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 interface has the fastest theoretical throughput: up to 10GBps. It remains to be seen who adopts Thunderbolt after Apple, but Intel will be building Thunderbolt into future motherboard models,along with integrated USB 3.0.. The first Thunderbolt drives we've reviewed have been promising: the Promise Pegasus R6 ($1,999 list, 3.5 stars) and the LaCie Little Big Disk Thunderbolt (240GB SSD) ($899.99 list, 4 stars) have broken all of our speed records, but all that performance comes at a steep price: these multi-drive arrays are best suited to high end graphics professionals. The Seagate GoFlex Thunderbolt Adapter ($99 list, 4 stars) ($1,999 list, 3.5 stars) is a much more reasonable solution that works with Seagate's modular GoFlex portable drives. AMD has announced a competing technology called "Lightning Bolt", but it remains to be seen ho 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 (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. USB 2.0 and FireWire 800 are more common than USB 3.0 or Thunderbolt, and those older interfaces work fine with a 5,400rpm drive. If all out speed is your goal, multiple drives (7,200rpm, 10,000rpm, or SSD) over Thunderbolt is the fastest (and most costly), with a single SSD connected via Thunderbolt or USB 3.0 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. Hands-off backup drives like the Rebit, Clickfree, and Seagate Replica are good choices for people who hate installing and configuring utility software. 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, packed in software isn't as important. Warranty is also an important 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.
For the latest news and reviews of external hard drives, check out our Hard Drive Product Guide.