Experts at TomsHardware.com state unequivocally that all hard drives will eventually fail, whether it's an internal or external hard drive. Because of that, an external hard drive should most definitely be part of a larger backup and storage plan -- not your only repository for important, irreplaceable data files, photos, video or other information.
If you are storing crucial, irreplaceable data on an external hard drive, experts recommend creating two backups. Storing identical data on two different drives virtually eliminates the chance that you could lose all your data following a drive failure. You can use internal or external drives in a RAID (redundant array of independent disks) to create backups, or you can use any combination of backup methods. Some higher-end systems come with multiple drives inside specifically for this purpose: for example, the PC-and-Mac-compatible Western Digital My Book Studio II (*Est. $180 for 2 TB) has two drives that are easily set up to mirror each other.
Even if you opt for a rugged external drive, such as the ioSafe SoloPro (*Est. $240 for 1 TB), storing a few DVDs or USB flash thumb drives with your most valuable work away from your computer can preserve that data if your computer equipment is stolen or destroyed by fire or another disaster. You can learn more about USB flash drives in their own report. Another alternative is an online backup service, and ConsumerSearch has a report on those as well.
External hard drives come in two main forms. The highest-capacity hard drives can be about the size of a book, with capacities up to 3 terabytes (TB). These high-capacity drives aren't portable -- they're meant to be parked on your desk. They rely on an external power supply. As a generalization, bigger drives are slower than smaller drives.
Portable external hard drives are about the size of a small paperback book or a deck of cards. These smaller drives usually connect with a USB cable and come in capacities up to 1.5 TB. They are small enough that they can usually pull all the power they need through the USB port, and you don't need an external power supply.
The disk's rotational speed partly determines the speed of data transfer. Reviews say 5,400 RPM is the slowest disk speed you should consider. Portable hard drives generally run at this speed, as do most laptop hard drives. A speed of 7,200 RPM is the most common for larger external hard drives. You'll also find faster external hard drives, but they are more expensive than 7,200 RPM drives. Drive speeds at this level won't matter much if you're using a USB 2.0 connection (the most common kind on all but the newest PCs), since the bottleneck is the cable and ports, not the drive itself. For USB 3.0 connections, drive speed becomes much more important.
Buffer size represents the amount of cached (stored) memory a drive can handle while waiting for the next request from the system. Bigger buffers can hold more data and deliver it more quickly. Budget external hard drives tend to have a 2 MB buffer, although 8 MB is common, too. Opt for the latter if you can, as it can make a noticeable difference. Higher-end drives have 32 MB buffers.
Most external hard drives connect to your computer through USB 2.0, USB 3.0, FireWire or eSATA ports. Your choice might be limited by the type of port you have available and the type of system (Mac or PC) you're running. However, if you have more than one type of port, you'll need to decide which type of connectivity to use. Certain ports offer faster performance -- but keep in mind that USB 2.0 (the most common connection, and usually the cheapest) works plenty fast for most users' ordinary backups, experts say.
Thunderbolt is the newest technology, though few at present are able to take advantage of that. Developed by Intel with help from Apple, Thunderbolt interfaces began appearing on Apple laptops and computers in February 2011. However, peripherals -- including external hard drives -- that use that interface have been slow to appear. Thunderbolt offers the fastest data transfer of all, with 10 gigabit per second (Gbps ) speeds -- theoretically twice that of USB 3.0.
USB 3.0, on the other hand, is theoretically 10 times faster than USB 2.0 (it's a toss-up with eSATA, discussed below), though not every USB 3.0 drive is able to take full advantage of that speed boost. For now, USB 3.0 is available only on some PCs. Microsoft estimates that 90 percent will have USB 3.0 by 2013, with the rest falling in line by 2015. USB 3.0 is backwards-compatible, so you can use a USB 3.0 drive on your USB 2.0 computer (you'll get the slower USB 2.0 speeds, of course). Initially, it was thought that Apple was not supporting the USB 3.0 interface, though rumors have surfaced that future products might, indeed, include USB 3.0 ports.
Although the theoretical bandwidth for USB 2.0 is higher than that of FireWire 400, actual transfer speeds are comparable. Some Mac Pro computers include the faster FireWire 800 interface (which is twice as fast as the original FireWire 400 standard, 400 megabits per second versus 800 megabits per second), and some external hard drives have this connector.
One of the advantages of FireWire is its support of what's called isochronous data transfer. This unwieldy term means that FireWire is an excellent choice for multimedia files, where uninterrupted transfer of time-critical data and just-in-time delivery reduces the requirement for costly buffering. Most Mac owners will use a FireWire connection, because most Mac computers come with at least one FireWire port.
External serial advanced technology attachment (eSATA) is a variation of SATA (SATA II is now the standard interface for internal hard drives). eSATA allows faster transfer speeds. Whereas USB 2.0 and FireWire have theoretical data transfer rates of about 400 to 480 megabits per second, eSATA can transfer data at up to 3 gigabits (or 3,000 megabits) per second. Real-life speeds are much slower, experts say. Although USB 3.0 has a theoretical transfer speed of 4.8 Gbps, experts say eSATA external hard drives still work a bit faster in real life.
The catch is that unless your computer has an eSATA or USB 3.0 port, you'll have to install an upgrade card on your computer. Some versions of hard drives come equipped with a PCI-card adapter, along with instructions for installing it. These cards might only work with certain types of computers (such as desktops), so read carefully before buying. Some only work with the brand of external drive they come with.
Although external hard drives shouldn't be your only lifeline should disaster strike, they play an important role in any comprehensive backup strategy. Reviews say the following about shopping for an external hard drive:
If you are so inclined, you can build your own external hard drive by buying an internal hard drive and enclosing it in an external hard drive chassis. We found more reviews for enclosures than drives, but this approach is more for do-it-yourselfers who may already have extra drives on hand. It is not cheaper, and if you want or need backup software, you won't get it. Some external enclosures have space for more than one drive. Add two or more 3.5-inch SATA II hard drives, and you have a RAID solution.