How to Select an External Hard Drive

by Steve McDonnell

External hard drives provide a convenient way to transfer files between computers, add additional storage to a computer and back up computers on your network. Before you select an external hard drive, determine how you plan to use it and prioritize the features you need. Then compare drives based on their portability and security, the amount of storage capacity they have, the type and speed of the connection they provide and the cost per megabyte of storage.

Selection Criteria

Your reasons for buying an external hard drive will help determine the features you need and narrow the selection of drives to choose from. Make a list of the different ways you plan to use an external drive and prioritize portability, security, capacity and speed. For example, if you plan to regularly transfer files between home and school or work, portability may be most important. If you're planning on using the drive to back up multiple computers, size may be critical. If you'll be using the drive as additional storage for a highly utilized computer, speed may be first on the list.


Decide if it's important to select a portable external hard drive. Portable drives are usually small and draw power from the USB connection. A stationary drive is usually larger and often requires a separate power source. Although portable drives are manufactured with some amount of shock resistance, most drives cannot withstand being dropped or exposed to water. Because of the risk of accidental damage to a portable drive, you have a higher risk of losing data. Some portable drives include encryption tools, which might be an important feature if you are concerned about the the drive being lost or stolen.


External hard drives range in capacity from less than 100MB -- for portable flash drives -- to 6TB or more. Estimate the space you need based on how you'll use the drive. For example, a 3TB drive can store about 3,000 hours of video, 750 two-hour DVDs, 960,000 pictures or 49,980 hours of music. Calculate the cost per megabyte for each drive to use as a basis for comparing cost and capacity. USB sticks are the smallest, most durable and most expensive per megabyte because they are made from flash memory and have no moving parts. Portable drives are typically more expensive per megabyte than stationary drives.

Rotational Speed

The rotational speed of an external hard drive influences how fast the drive performs. Most external hard drives and laptop drives operate at 5,400 RPM. Many internal hard drives operate at 7,200 RPM, which makes drive reads and writes faster. While there are 7,200 RPM external hard drives, they are harder to find and more expensive than 5,400 RPM drives. If you're connecting a drive using USB 2.0, you won't notice any difference between a 5,400 RPM and a 7,200 RPM drive.

Transfer Rate

The speed of an external drive is typically most influenced by the type of connection it has with the computer. A drive connected to a USB 2.0 port has a top speed of 480 Mbps. A USB 3.0-compatible drive connected to a USB 3.0 port has a maximum speed of 5 Gbps -- ten times greater -- and is more efficient in the way it transfers data. An external drive connected using an eSATA connection renders it less portable, but provides speeds comparable to USB 3.0. If speed is important, select the fastest USB connection provided.

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