An external hard drive can do several jobs in your home or office. It can provide a place to back up your files so they won't be lost if your computer's internal hard drive crashes. It can also expand your computer's storage capacity, giving you more room for large files such as digital video, high-resolution photos and music collections. Finally, these drives offer a simple way of sharing data between computers by simply unplugging the drive from one machine and plugging it into another. They're typically faster than online backup services (which are covered in their own report), and they allow you to keep your files under your control, rather than entrusting them to strangers.
External hard drives can use two different types of technology. Hard-disk drives (HDD) are mechanical, writing to and reading from spinning platters. This type of drive offers the advantage of higher storage capacity at a relatively low cost per gigabyte (GB). Solid-state drives (SSD), by contrast, use flash memory in place of spinning disks. Most SSD drives are internal, but external drives that use flash memory are becoming more available. SSDs are significantly faster than HDDs, as well as quieter and more compact. In addition, their lack of moving parts makes them less vulnerable to physical damage. On the downside, they're also considerably pricier, and at present, no consumer model can store more than a terabyte (TB) of data.
Different external hard drives excel at different tasks. Portable hard drives are designed to let you take your files on the go. They're compact, relatively inexpensive (about $75 for 1 TB of storage) and able to run off your computer's battery via a USB connection. Desktop hard drives are bigger and pricier (anywhere from $100 to $350), but they also come in larger capacities, making them more suitable for storing giant multimedia files. Some portable and desktop drives are ruggedly built to protect the data inside from all kinds of physical damage, from falls to floods.
Some drives specialize in working with more than one computer (or other device) at a time. Network-attached storage (NAS) drives can back up and share files across multiple computers on the same network. Wireless hard drives can add storage without the need for a physical connection; most can also stream content from the Internet to your devices via Wi-Fi. Wireless external hard drives cost more per TB than either desktop or portable drives; the best-rated 1-TB drives are in the $200 range.
To identify the best hard drives in each category, we consulted reviews from a variety of computer publications and websites, such as PCMag.com, CNET and Macworld. These sources conduct detailed, in-house tests of external hard drives to determine how fast they are at reading and writing various types of files. Reviewers also talk in detail about usability, evaluating the setup process, the software that comes with the drive, and the ease of using it with various types of third-party software. In addition, they offer comparisons of value, noting how a particular hard drive compares to others in its price range.
The one factor that professionals can't always evaluate is long-term durability. In the test lab, they can't usually spend a long enough time with an individual drive to see how it holds up over weeks or months of use. Thus, we turned to user reviews from retail sites such as Amazon.com and Newegg.com for information on reliability. By looking at the hundreds or, in some cases, thousands of individual reviews these sites provide for a specific model, we were able to spot those with a pattern of poor reliability.