Why choose an external hard drive?

Installing additional internal hard drive space on a desktop PC is usually cheap and easy (most PCs have one or more spare internal drive bays). But that extra internal hard drive space won't be portable. You'll also need to get someone to install it for you if you're not confident doing it yourself. And, unless you have mirror hard drives, that extra space won't be useful for backing up a computer.

Laptop computers can't have internal hard drive space added because there's simply no space.

External hard drives are portable and can be used on any computer as long as they have a USB, FireWire or eSATA connection (see 'Connection type', below). Installation is simple – just plug it into the computer. They can also perform regular backups so that when your computer dies (and eventually all computers do) you won't have to deal with the pain of losing years of data.

What's available?

There are 3 main types of external hard drive - portable, desktop and NAS.

Portable drives
These use similar hard drives to those in laptop computers, generally 2.5 inch. They're light, small enough to be transported and range from around 160 to 500GB of storage space. They're typically powered through the USB port, and some have a USB power-sharing cable or USB/power combo cable, which means there's less to carry when you transport the drive.

Portable drives are usually made with a rugged, shock-resistant enclosure since they're more likely to take knocks than desktop drives.

Desktop drives
Most home users opt for a desktop drive as they're simple to set up, easy to use and don't require any networking knowledge. They use 3.5 inch drives (the same type generally used in desktop computers). They weigh a kilogram or more, so are not as easily transported as portable drives. But they're considerably cheaper per megabyte than portable drives and tend to have a larger capacity - up to several terabytes.

Unlike NAS drives (see below), desktop drives can only be accessed by one user at a time. They're powered by an AC adapter.

NAS
A Network Attached Storage (NAS) drive is mostly used for file sharing and additional storage space. They can be connected to a router via an ethernet connection and allow multiple computers on a single network to share storage. They're often used in small businesses, but multiple users on a home network use them for a central storage unit or for backing up all the computers.

If you're looking for a hard drive to be portable, compact and used in various locations, opt for a portable or desktop based hard drive. But if you want a network hard drive to be kept in a single location, and desktop real estate isn't a factor, a NAS based drive would be a better option. NAS drives typically weigh 1-5kg, whereas desktop drives only weigh 1-2kg. Don't forget the look and style factors as well.
 

Specifications and features

Capacity

How much hard drive space do you need? The table below shows how much you can store on a hard drive.

Storing 280,000 photos or 250,000 songs may seem unlikely, but the figures illustrate the vast amount of data that can be stored. The larger drives are suitable for people doing lots of photo- or video-editing - if you're using high definition video a 1TB drive will only store about 120 hours of content.

Get more space than you think you'll need, especially if you're planning on using the same computer for several years.

Revolutions per minutes (RPM)

The higher the RPM, the faster the drive can read or write data. A common RPM for hard drives is 7200, with high-end drives having speeds of 10,000 RPM. Be wary of models that don't have a stated rotational speed.

Buffer

A drive's buffer is an amount of RAM on the drive to store frequently accessed data. The more buffer RAM on the drive, the more data can be stored in the cache to decrease the amount of physical drive operation. Most drives today come with an 8MB drive buffer. Some high-end drives come with a larger 16MB buffer.

Connection type

• USB is the most common connection for hard drives.
• FireWire (in speeds of 400 and 800) is sometimes used for external storage devices, and is generally considered faster than USB. FireWire can transfer data more than twice as fast as USB with some models – but not all.
• Another fast connection is eSATA. Many computers now come equipped with an eSATA connection.  If yours doesn’t, it's necessary to install either a PCI or PC card adapter that features the connector, or use a PCI bracket that converts an internal SATA connector into an eSATA connector at the back of the machine.
• NAS based drives use an ethernet port for connection across a network.

Media/card readers

These devices are typically used to read the data from a memory card such as Compact Flash (CF), Secure Digital (SD) or MultiMedia Card (MMC). Some drives feature these devices as standard.

Seek time

Seek time (the length of time taken to find a particular piece of data) is often considered a big deal in terms of specs, but it really makes little difference. It's measured in milliseconds with the lower the figure the better the seek time. Most home or small business users wouldn't notice the difference between seek times.

Warranty

A hard drive warranty generally only covers the failure of the physical hard drive - it doesn't include loss of data. The best idea is to mirror the backup to another drive.

Warranties range from 1 to 5 years. A long warranty may give you less trouble if things go wrong, although you still have rights under the Consumer Guarantees Act with a shorter warranty.