Some drives have more than one interface, which is convenient if you use the drive on more than one platform. All three interface types provide enough electrical power for most any portable hard drive, so you don't need to carry a power brick.
Some drives give you the option of ditching the cable altogether. These are particularly handy for streaming media to a computer, tablet, or smartphone. They produce their own short-range Wi-Fi hotspot and operate on battery power.
Most platter-based portable hard drives use 2.5-inch mechanisms, so they're generally just slightly larger than the drive itself (typically around 3 inches wide, 4 inches deep, and less than an inch thick). Cases are typically made from plastic (ding resistant), aluminum (lightweight), or a combination of the two. Western Digital and Seagate have lately started offering dual-drive products that are necessarily twice as thick as their single-drive cousins.
Some manufacturers offer optional external cases to protect their drives. The best ones have a cable pass-through so you don't need to take them out of the case to use them.
If you're security-conscious, consider a hard drive that must be unlocked before you can use it, or one that automatically encrypts the data stored on it (and decrypts it when accessed by an authorized user). Most drives with "secure" branding do both. Some drives use numeric keypads to authorize access; others have fingerprint readers.
Defects happen. Make sure the drive you buy is covered by a solid warranty. Seagate offers a limited warranty of two years on most of its portable drives. Western Digital covers its Passport drives with a three-year limited warranty.
Higher-capacity drives cost more than lower-capacity drives. Duh. The best way to evaluate a drive's price tag is to measure its cost per gigabyte. A 2TB drive priced at $120, for instance, would have a cost-per-gigabyte of 6 cents (most hard-drive manufacturers advertise 2 terabytes to be equal to 2000 gigabytes, even though 2048GB is more accurate). Price-to-performance ratios are important, too. Portable drives based on SSDs will cost considerably more than those based on mechanical drives, but they'll also be considerably faster.
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Lots of portable hard drives pass through the PCWorld Lab. Each one is carefully measured, tested, and evaluated. We benchmark portable hard drives by measuring their read/write performance. We connect the drive to a testbed PC and copy a 10GB collection of small files from the PC's internal SSD drive to the portable hard drive (a write test). We repeat that test using a single 10GB file. We then copy the 10GB collection of small files from the portable hard drive to a folder on the PC's internal SSD (a read test), and repeat the test with a single 10GB file. Lastly, we run Crystal DiskMark with the Test Size set to 4000MB and report those results.
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