What's the difference between a good hard drive and a great hard drive?

If there's one thing computers never seem to have enough of, it’s storage space. High-definition media, high-storage optical discs and faster broadband speeds mean that it's easier than ever to fill up your hard drive. Luckily, it’s also cheaper than ever to get a bigger hard drive.

Hard disk drives are currently bigger and better value than at any point since their creation, and that makes buying one relatively easy. Any hard drive you buy today is almost certain to eclipse your current storage device in terms of capacity and speed, and it'll probably cost less while doing so. For that reason, you might be tempted to think that not a lot of thought needs to go into buying a hard drive.

Obviously, if that were the case, we wouldn't have much of an article for you. But here's the thing: there’s a big difference between buying a good hard drive and buying a great one. If you want to make sure you don't get lumbered with outdated, under-performing hardware, follow our advice so that you can pick the right device with the right specs.

Just so there's no confusion, we're only going to cover internal hard disk drives here. We've already done solid-state drives. And external hard disk drives will get their own guide in due course. As it is, internal hard drives may not be the fastest storage medium, nor the most portable, but in terms of all-round practicality and popularity they're still the industry's favourite.

So settle in, and by the end of this article you should know for certain what you're looking for in an internal hard drive and, perhaps more importantly, what to avoid...

How Much Should You Spend?

At the moment, you can buy a fairly serviceable 160GB hard drive for as little as £35. If you've had your computer for a few years, this might seem like a ridiculously large amount of space, particularly if you're only a light or casual PC user. You might also think that you could actually live with even much less space than that, if it means you're able to pay a lower price.

However, don't think that a smaller drive would cost you any less. It's tough for manufacturers and retailers to go any cheaper than f 35 on an internal hard drive, because the constant improvements in hard drive technology makes it so much cheaper and easier to pack in drive capacity that they have reached the point where it becomes economically unviable to make a drive any smaller.

Let's assume, though, that you're interested in something other than the bare minimum. Hard drives tend to become better value the more you spend on them. Using Seagate drives as a barometer, you can get a 250GB hard drive for around £45, a 500GB one for a mere £50, and despite having four times the capacity of that, a two terabyte drive can be bought for as little as £80.

The lesson, then, is to spend as much as you can afford when buying a new hard drive, because the storage capacity doesn't determine the price so much as the technology and workings that underpin it.

However, that said, there is an upper limit you should keep in mind. The most expensive 2TB hard drives cost around £400, and although such drives can offer faster speeds, hybrid SSD storage, better reliability and other extra features such as built-in encryption, they're designed for specialist use. If you're a home PC user with no other needs, you should be able to get the hard drive you want by spending between £50 and £100.

Certainly, if you don’t know you have a good reason to do so, you shouldn't spend any more than £150 on an internal hard drive. As with any component, it's tempting to buy extra features in the hope they'll pay off long-term, but remember that hard drives don't last more than a few years under normal use. so only pay for the features you need in the immediate to mid-term.

What Make/Model/Manufacturer Should You Look For?

There are plenty of hard drive manufacturers to choose from, but the differences from one to the next are generally minor in terms of actual technology. What matters most is build quality As components, hard drives are particularly prone to failure, so the reliability offered by a well-known manufacturer is one way of ensuring you don't get stung Warranties are all well and good, but they don't bring back lost data.

The best-reviewed manufacturers are typically Seagate and Western Digital, so don't be afraid to pay a little more for a drive created by one of those companies. Both have a selection of product lines, which offer different degrees of speed and fidelity, so here we'll explain the difference between them.

If you choose a Seagate drive, you will almost certainly want to buy from the Seagate Barracuda range, which is the company's line of standard 3.5" internal hard drives. There are other drives available, but these should be avoided unless you're building a PC which fits into one of the specific categories they're aimed at.

Seagate Momentus drives, for example, are thin, low-power 2.5" drives intended for use in notebook PCs. They're cheaper than Barracudas but also slower. Seagate Pipeline drives are designed for use in media centre PCs, and are optimised for quiet and low-power running. This makes them cost more than Barracudas, but they actually run a little slower and have poor random access times.

The most expensrve Seagate drives are in the Seagate Constellation line, which offer enterprise-level storage with self-encrypting storage, high-capacity platters and high integrity components to reduce the chance of failure or error. As suggested earlier, these are unnecessary features for home use.

For Western Digital, you should pay attention to the colour ratings of the drives. Any drive marked 'blue' is a medium-power, medium-performance drive, while drives named 'black' are high-power, high performance, and have a warranty period of five years instead of three like other Western Digital drives. 'Green' drives are, as the name suggests, engineered for low-power running, but they sacrifice speed and performance in favour of energy efficiency and cool running.

You may also see names such as 'Caviar' and 'Scorpio' applied to Western Digital drives, but all that denotes in practical terms is whether they're 3.5" (desktop) or 2.5" (notebook) models respectively.

Note that although Caviar Black drives are high-speed/ high-performance, they're still within the range that the Seagate Barracuda covers, and not the equivalent of the Seagate Constellation or other specialist drives For that reason, we recommend that any non-specialist user buying a Western Digital drive consider both Caviar Blue and Caviar Black.

What Technology Should You Look For?

Although hard drive specs list huge numbers of attributes, there are a few main ones to look for, which you can compare across multiple models for an accurate picture of performance. Capacity is undoubtedly the main attribute any hard drive buyer looks for, and if you're not a particularly informed hard drive buyer, it may even be the only one. However, it doesn't tell you anything about how a hard drive performs - certainly no more than knowing how many seats a car has tells you what its fuel efficiency and top speed are.

The first thing you should pay attention to after capacity is the connector type If you have an old motherboard, you may need to buy an IDE drive, but most recent computers will need a more up-to-date SATA connection. Don't worry too much if you buy the wrong one, converters are available, but if in doubt go for SATA because that way you won't be limited by the drive's connection speed.

The current SATA technologies are SATA-2 (or SATA 3Gbps) and SATA-3 (or SATA 6Gbps). SATA-3 is twice as fast as SATA-2, allowing a maximum data throughput of six gigabits a second, so in theory you should go for hard drives with that sort of connection. However, it's not actually that obvious a decision, because mechanical hard drives can't physically achieve the kinds of speed it would take to saturate a SATA-2 connection, let alone SATA-3, so the difference is more academic than practical. If there's a SATA-2 drive you want and it's cheaper, don’t be afraid to buy it.

If you're interested in getting the best access speeds out of a drive, a more important statistic is what RPM it can achieve. RPM refers to the number of revolutions the platters can make per minute, and the higher this value, the lower the latency between data being requested, read and delivered. The slowest you'll see current drives run at is 5400rpm (giving a 5.55 millisecond latency), but the minimum we’d advise readers to consider is the next step up: 7200rpm (4.16ms latency). It's possible to get drives that run at 10,000rpm (3ms latency) or even 15,000rpm (2ms latency), but such drives are pushing the realm of specialist equipment and will undoubtedly cost a premium.

Finally, pay attention to the amount of cache on a drive. The data cache on hard drives is a temporary storage area where data can be placed so that the disk does not get delayed waiting for the physical act of a read/write to happen. The more cache you have, the better your drive's performance will be in practical situations.

Mobile drives tend to have a very small amount of cache (normally 8MB) as a cost-saving measure.

Desktop drives normally have 32MB, but higher-end drives may have 64MB. Be aware that while more cache is better, it's hard to quantify the performance of extra cache in very precise ways, as it will affect operations at speeds that are imperceptible in isolation For that reason, if you’re looking to cut corners on price, going for a drive with a 32MB cache instead of 64MB cache will most likely not affect the speed of your computer unless you frequently run very hard drive intensive tasks such as video-encoding.

Is Now The Right Time To Buy?

Now is probably a better time to buy a hard drive than at any time in the last 12 months. A series of natural disasters in 2011 dramatically affected the traditional centres of hard drive production in both economic and practical terms, sending hard drive prices into a huge spike They have since almost completely normalised, and they certainly aren't expected to drop a significant amount in the near future.

It's also worth noting that while solid-state drives perform vastly better than mechanical hard drives, the price still hasn't reached the point where they're a totally viable alternative. They may be getting more and more popular by the day, but it'll be a year or two still before a 500GB SSD drive can be picked up for the amount a 500GB hard drive currently costs. With that in mind, now is definitely a good time to buy a mechanical hard drive, because it’s unlikely you'll be replacing it with a solid-state drive in the short or medium term.

That said, in all probability any hard drive you buy in the next 12 months may well end up being the last one you ever need - so don't worry too much about the long-term practicality of your choice. SSD prices are dropping fast, and the longer you wait before buying a hard drive, the less sense it'll make to buy one at all.

What Are The Technical Constraints?

As with any internal component, there are space and power requirements for hard drives. Since you already have a hard drive in your PC, however, it's a safe bet you can fit at least one more in. Most motherboards have two SATA/1 DE connectors intended for hard drive usage, and you should have power supplies for at least two hard drives as well (although it's always worth checking).

The most likely technical problem you'll encounter when using a hard drive is the possibility of a hard drive crash. While mechanical hard drives aren't especially unreliable in absolute terms, they do fail far more than any other component you're likely to put into your computer Luckily, any hard drive with a manufacturing defect will fail pretty quickly - usually within the first three months of use - so we recommend you keep your original drive running or stored in a safe place for at least that long.

If your disk survives this initial period, you can be fairly certain of a year or two of completely error-free use before the drive enters the 'wear-out' phase, at which point the risk of failure will steadily increase in line with the drive's age.

Despite this, all you need to do to maintain your drive is leave it alone (physically speaking) and run an occasional disk check, replacing it as soon as bad sectors begin to appear.