Hard drive buying guide

We walk you through the process of buying a new hard drive, and explain all the jargon: from network-attached storage to solid-stated drives

NAS devices are not suitable for all users. They're most suited to people who need to regularly share large amounts of data between computers and conduct frequent backups.

Form factors

Internal PC drives typically come in the 3.5in form factor, which means they fit in the same size slot as a 3.5in floppy drive. There are other sizes, however.

The next most common size for hard drives is 2.5in. These are considerably smaller and lighter than 3.5in drives, and usually have less capacity. While 2.5in hard drives tend to offer slower spin speeds than 3.5in drives, they can actually be faster, since the drive head has less distance to travel over the platter. They are usually designed to go into notebook computers, but it is becoming more common for them to be sold at retail for use as external drives because they are light and don't require much power. A single USB or FireWire port is often enough to power a 2.5in drive.

1.8in hard drives are also quite common, but they are not generally found on store shelves. The drives are typically used in devices such as iPods. Some companies use 1.8in drives in slimline notebooks.

Microdrives, or 1in drives, are available, but are not as common as the other form factors. They are largely irrelevant today, but they once offered up to 8GB of storage in the same form factor as the CompactFlash cards frequently found in D-SLR cameras. CompactFlash cards now surpass Microdrives in capacity, as do other types of flash storage such as SD cards.

Internal or external?

If you want to expand your computer's storage, then you will need to decide whether to add an external hard drive or an internal one.

Internal hard drives are more complicated to install because you need to open up your computer's case. If you are replacing your primary hard drive, which typically contains the operating system and applications, you will have to reinstall and/or copy your programs, operating system and data files to the new drive.

You may want to add a secondary or even tertiary internal hard drive. Adding an internal hard drive lets you take advantage of the faster speed of SATA2 (compared to USB 2.0 or FireWire), although an external hard drive that supports eSATA will also offer fast file transfers.

You can also purchase a drive caddy that lets you turn an internal hard drive into an external drive. The dock or caddy will have one or more ports (USB 2.0, for example) that let you connect to a computer.

External hard drives are slower than internal drives in most cases, but they have the benefit of being highly portable and easy to set up. External hard drives are often preformatted to a specific file system. This means if you buy one with a file system that is supported by your operating system, you won't have to fiddle with any settings.

Hard drive specifications

As well as storage capacity and the type of interface used, there are a range of other specifications that affect the speed, durability and reliability of a hard drive.

RPM

The most frequently quoted specification, apart from the capacity of a hard disk and how it connects to the PC, is spin speed of a drive. This is measured in revolutions per minute (rpm). The faster the drive platters spin, the faster they carry data to the read head. All other things being equal, a 7200rpm drive should be able to stream data off a platter 33 per cent faster than a 5400rpm drive. The lion's share of consumer 3.5in hard drives available now are 7200rpm, but some 2.5in and older 3.5in drives run at 5400rpm.

Drives with speeds of 10,000rpm or faster tend to be considerably more expensive and have much smaller capacities than 7200rpm drives. They are typically used for drive-intensive functions, such as video editing. However, some computer enthusiasts purchase these drives to give their systems a little extra edge.

Western Digital does not disclose the spin speed of its Caviar Green line of internal hard drives, as the company believes it does not accurately indicate the drive's speed due to the power-saving technologies employed. However, most tests show that these drives average a spin speed around 5,400rpm.

Disk cache

Also called the disk buffer or cache buffer, the disk cache is a small amount of memory (relative to the drive's storage capaicty) that acts as a buffer between the hard drive's main components and the computer itself. A microcontroller on the drive holds the memory — usually between 16MB and 32MB on modern hard drives — and uses it to store commands from the computer demanding information from the hard drive.

The size of the cache is often relative to the drive's capacity; a 2TB drive may have a 32MB disk cache whereas a 250GB drive in the same range may only have a 16MB cache. Having a larger cache buffer means that the drive can store frequently used data in its fast local RAM before writing it to disk. With a smaller buffer, the drive is limited in the amount of data it can store and often needs to look for it in the main system RAM, which can slow the process down.

Native command queuing (NCQ)

SATA2 introduced an advanced buffering technology known as native command queuing (NCQ), which minimises the movement of the hard drive's head and thus reduces the access times.

Native command queuing will dynamically reorder data fetch instructions to maximise the efficiency of the hard disk. Say the computer asked for some data that was on track 20, then some data on track 300, than some data on track 30. A normal (non-NCQ) hard disk would do these sequentially, moving the head from one side of the disk to the other. A hard disk with NCQ, working with a supporting chipset, would instead execute operation three ahead of operation two, to save the head from travelling all the way across to track 300 and then back again.

It's hard to quantify the speed improvement that NCQ provides, since it largely depends on the conditions. According to hard drive vendor Seagate, it can, under the right circumstances, provide an overall speed boost of 30 per cent or more.

Seek/access time

A hard drive's access time is the length of time it takes to start reading a given piece of data. It incorporates the amount of time it takes for the drive to move the head over the track on which the desired data is stored (the "seek time") and the amount of time it takes for the rotation of the disk to carry the part of the track on which the data is stored around to the head (the "latency"). The average seek time plus the average latency will give you an indication of the average access time of the hard disk.

The access time is important, especially if the hard disk has to access lots of small or fragmented files that are scattered around the platter. Often it will take longer for the hard disk to get the head in position to read the data than it will to actually do the reading.

Hard disk manufacturers don't normally release this information, so discerning consumers will have to go digging for it. Typically, seek times are in the 8-10 milliseconds (ms) range.

MTBF

MTBF (mean time between failure) describes the mean time it takes for an error to occur in a hard drive model's operation. This is a figure used by manufacturers to advertise a drive's reliability and durability.

Because of the way the vendors calculate it, however, MTBF is largely a useless measure. Figures in excess of 1,000,000 hours are commonly quoted, which would lead many to believe that the hard disks, on average would last a million hours or more before breaking down. That's not the case.

Warranty

Instead of relying on MTBF figures, the best way for most users to determine a hard drive's reliability is to look at its warranty. Since hard drive are expected to eventually fail, almost all hard drive manufacturers offer a hassle-free approach to pursuing a Return Merchandise Authorisation, which usually leads to the manufacturer replacing the drive without cost.

When purchasing a hard drive, you should take note of the warranty term. Most hard drives have a 3-5 year warranty, during which time hard drive manufacturers will offer an RMA in case of drive failure, either through an online process or by returning the drive to the store of purchase.

Tags solid-state drivesnetwork-attached storagehard drivesbuying guidestorage

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PC World Staff

PC World

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