Portable hard drives are smaller and easier to tote around than typical external desktop drives. Some feature case designs that reduce impact shock.
With portable hard drives, USB is king, though there are FireWire and eSATA options (Macs do not come quipped with eSATA ports). Some drives are bus-powered, which means they draw power from a FireWire or USB connection on your laptop, with no power adapter required. But some older laptops do not supply enough USB bus power, so you’ll have to either use a special cable that connects the hard drive to two USB ports or use the drive’s power adapter.
There’s always the chance that you’ll lose your hard drive, so it’s wise to secure your data. Data Locker offers portable USB 3.0 hard drives that feature hardware encryption options and a numeric touch screen where you enter a PIN to unlock the drive. The LaCie Rugged Safe ($200 to $800) provides secure storage through hardware encryption and fingerprint authorization.
If you don't want to spend extra money for a secure hard drive and you have Lion on your Mac, you can encrypt a standard portable hard drive using Disk Utility. There’s also third-party software, like the free TrueCrypt, that encrypts content on just about any external drive. Though it’s not as secure as hardware-based encryption, software encryption can stop all but the savviest hackers from accessing sensitive data.
Portable hard drives buying advice
Capacity Capacities range from 160GB to 1TB. If you have lots of media or graphics files, more is better. More capacity usually means a bigger bite out of your wallet.
Hard disk or SSD Hard drives, the mechanisms most people use to store data, have disk platters that spin inside the case. A solid-state drive (SSD) uses microchips and has no moving parts. SSDs are much faster than disk-based drives, but they are more expensive. SSDs also don't come in the large capacities that you'll find with disk-based drives.
People who need the fast data throughput (video editors, multimedia creators, audio editors) will benefit from SSDs. Most people will do fine with a regular hard drive.
Connectivity You’ll use USB or FireWire 800 to connect to you Mac. Many portable hard drives are equipped with USB 3.0 ports. While the Mac uses USB 2.0 ports, you can still use USB 3.0 drives on USB 2.0 ports, but you won't get the speed increases USB 3.0 offers. Many drives feature multiple ports, but some offer only one (usually USB or FireWire 800).
USB is a must if you’re sharing the drive between Macs and PCs, and it’s your only option if your Mac doesn’t have FireWire.
If you bought a Mac released this year, it probably has a Thunderbolt connector. There's only one portable Thunderbolt drive that we are aware of, the LaCie Little Big Disk Thunderbolt Series. Keep in mind that this drive has only Thunderbolt connectors, so you can't connect it to a Mac that doesn't have Thunderbolt.
Speed The faster the platters in a hard drive spin, the better the performance is. Most portable hard drives feature 5400-rpm drives, but faster 7200-rpm drives are appearing. SSDs are much faster than hard drives, but also more expensive.
Durability Falling off tables, surviving coffee spills, getting tossed with the crumbs at the bottom of your bag—these are just a few things portable drives face while serving duty. If you’re accident-prone, look for case designs that reduce impact shock, prevent sliding, and provide protective port covers to prevent dust bunnies from wreaking havoc on your connection.
Added features Almost all USB or FireWire external hard drives are compatible with Time Machine, as long as the drive is HFS+ formatted. A drive specifically branded as one for the Mac is usually preformatted using HFS+. It's the drive isn't formatted for the Mac, you can format it using Disk Utility.
Some drives include extras such as bundled software, one-touch backups, software encryption, and carrying cases.
Our favorite portable hard drives
The WD My Passport Essential (Read our full review. [$130 for 500GB (Get best current price for the WD My Passport Essential); Western Digital]) is a portable hard drive that fits easily into your jeans pocket. It’s simple to use, really quiet, and offers read speeds that closely rival other drives in its category.
Editor's note: This is an updated version of a previous article that includes the most up-to-date information.
[Kris Fong, Roman Loyola, and Mauricio Grijalva contributed to this guide.]