EmailHard drive form factors, speed and capacities
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 2 terabytes (2,000 GB). 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 500 GB. 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.
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 16 MB buffers.
Connectivity: FireWire vs. USB vs. eSATA
Most external hard drives connect to your computer through USB 2.0 (backward compatible with USB 1.1) or FireWire ports. Your choice may 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 both types of ports, you'll need to decide which type of connectivity to use. 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, 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. Owners of lower-end PCs will generally go with USB 2.0, but higher-end PCs have FireWire and eSATA ports. FireWire is now fading from popularity. It is likely to be useful only if you have FireWire ports on your current computer(s).
The newest connectivity method is called eSATA. The acronym stands for External Serial Advanced Technology Attachment and 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 typical 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.
The catch is that unless your computer has an eSATA port, you'll have to install an upgrade card or bracket on your computer. Some eSATA hard drives include a PCI-card eSATA adapter with the drive, along with instructions for installing it. Portable eSATA hard drives are rare. Most eSATA drives support Windows (2000 Pro or higher) and Mac (OS 10.4 or higher) computers.
Most external hard drives with eSATA ports also have USB ports. These drives are the best long-term solutions. Even if your computer doesn't have eSATA ports and you don't intend to add them, your next desktop or laptop computer is very likely to have them. You'll be able to capitalize on the performance enhancement then. In theory, an external drive with eSATA provides the fastest data transfer, but reviewers often find it isn't much faster in reality.
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:
- Choose an external drive based on your source system, operating system and backup needs. If you mainly plan to store or back up data files, you don't need the fastest external hard drive. Those who plan to back up multimedia files need more storage capacity and faster transfer speeds. Mac users should gravitate toward FireWire 400 or 800 instead of USB 2.0. If you want an eSATA drive, you may need an adapter for your computer.
- Consider preloaded, easy-to-use backup capabilities. You may benefit from software that makes backups easy so that you back up often. Some models build this into the hardware design by providing one-button backup capability. However, experts and owners often aren't enthusiastic about the backup software that ships with external hard drives. You do not need to use a bundled program for backup. Any backup software will work.
- If your storage needs are modest, consider a USB flash drive instead. Because it's solid state and doesn't have the mechanical vulnerabilities of platters wobbling at 7,200 RPM, flash technology is the safest and most practical storage medium. In practical terms, current capacity tops out at 128 GB; though larger 256 GB drives are becoming available, they are prohibitively expensive. See our companion report on
USB flash drives .
- Assume that your drive will eventually fail. Reviewers uniformly agree that an external hard drive should be part of an overall backup strategy, which should also include backing up to removable media or storing important files online.
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.