HOW TO BUY AN EXTERNAL HARD DRIVE

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, including 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. Notebook-class (aka pocket) hard drives, like the Toshiba 650GB External Hard Drive and Western Digital My Passport Elite, 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 2 Terabytes (TB) per mechanism, but some drive makers put two to four mechanisms into a drive chassis for more storage (i.e., two 2TB drives equal 4TB of storage). Notebook-style drives have recently reached 1TB per mechanism, but capacities like 640GB or 500GB are more common.

External solid-state drives (SSDs) have appeared on the scene, mostly in the notebook-style form factor, but these are still rare because they're pricey in terms of cost per gigabyte. They're currently limited to smaller capacities, specifically in the 64GB to 256GB 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) to connect the SSD to your system, since the transfer rate of eSATA is inherently so much faster than USB. An eSATA SSD might eventually be worth it if the prices come down over the next few years.

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 just came to market in late 2009, and iSCSI is mainly used on professional-grade drives like the Drobo Pro. USB 3.0, becoming the port of choice, provides faster transfer speeds like in the Buffalo Drive Station, which was able to transfer a 1.4GB file in 28.4 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 fastest interface you'll likely 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 the fastest, it does not provide power over the connector cable and will require either a USB cable for power or an external AC adapter. It's also currently the most expensive. USB 3.0 is purported to be even faster than eSATA, but so far that interface is barely newborn and won't impact consumers until late 2010 at the earliest. 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 Bata at the slower USB 2.0 speeds). You can find drives with multiple ports (for example a triple interface drive with USB 2.0, 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.



Is Drive Speed Important?

Some drive manufacturers will crow about the speed of their drive mechanisms. While it is true that a 7,200rpm drive is inherently faster than a 5,400rpm drive, you will only see an improvement in throughput if the drive is connected internally to the PC on the motherboard, or with an eSATA cable. USB 2.0 and FireWire 400/800 interfaces aren't fast enough to handle the raw throughput of a 5,400rpm drive, let alone a SSD, 7,200rpm, or 10,000rpm drive. I'd only worry about the rotational speed of the drive if you're eventually going to remove that drive and pop it in your laptop or desktop (like in a backup/recovery/upgrade kit). 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 Toshiba 650GB are good choices for people who hate installing and configuring utility software. Warranty is also important: 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.