The price of storage has plummeted since the 80s, but it's still important to choose the right drive for your needs. Digit outlines your options.

Back in the early 80s, a hard drive would have set you back around £300 - for a meagre 20MB capacity. Now, you can pick up a 200GB hard drive for around £150. That's 10,000 times the storage for half the price. However, there's a lot more than size to consider when shopping for today's hard drives.

Inside or outside?

If you don't need your operating system to boot from the drive, you might want to consider buying an external hard drive that connects to your PC via FireWire or USB 2.0. Compared with internal drives, they're far easier to install - you just have to plug them into the front of your computer's case. They're also easy to move to another PC, and they keep excessive heat out of your system's innards.

But external drives cost up to twice as much as their internal counterparts. They move data much more slowly than internal ATA drives, too. If you frequently move massive amounts of data between your applications and your hard disk, you probably want to use an internal drive.

An internal model requires space. Before you shop for a new drive, check for a free drive bay inside your PC. Jamming the drive into your case may restrict airflow and cause overheating.

SATA or PATA?

There are two interface standards to choose between as well - the new Serial ATA interface and the time-honoured Parallel ATA, or EIDE interface. SATA drives offer faster data transfer speeds and vastly easier installation, among other advantages. However, a SATA drive isn't necessarily the obvious choice.

You'll pay a 20 to 40 per cent premium for a SATA drive, and your system's motherboard will need to have SATA connectors - if your PC is more than two years old, it probably won't be compatible. Most new PCs and motherboards come with both SATA and PATA connectors. If your system doesn't have SATA connectors, you can add them via a PCI host adaptor card.

The SATA interface bumps up your drive's maximum data transfer speed to 150mbps from current PATA limits of 100mbps or 133mbps, but this won't affect the performance of most PCs. The majority of hard drives have a maximum sustained data transfer rate closer to 80mbps. Unless you're constantly moving huge files, that slower transfer rate should be fast enough for your needs.

On the plus side, SATA installation is a breeze. The 8-wire SATA cables are far narrower than the thick 40- or 80-wire PATA cables that tend to clog PC interiors. Each SATA drive connects exclusively to one SATA connector, so you needn't set jumpers to master/slave or struggle with any of the many other PATA configuration hassles.

Scoping the specs

Before you buy, you'll want to familiarize yourself with the various specifications associated with hard drives. The first number you'll encounter when shopping is the drive's rotation speed. Hard drives rotate at 5,400, 7,200, or 10,000 revolutions per minute (rpm). Most mainstream hard drives spin at 7,200rpm, but choosing a 5,400rpm drive will save you a few quid.

Like your CPU, your hard drive uses buffer (or cache) memory to speed up data retrieval. Most drives come with either 2MB or 8MB of cache memory. 2MB should be fine for handling standard PC applications; but for work with image or video files, performance will be noticeably better with 8MB of hard-disk cache.

The drive's access time is a measure of how fast the drive locates a given file stored on it. Faster is better, of course, especially if you frequently move hundreds of small files at a time.

Another significant variable is the drive's data transfer rate. However, comparing the vendor-supplied data transfer rates of different hard drives can be tricky, because there are several methods of calculation. Stick to cache size and rotation speed as guides to a disk's performance.

Big problems

If you try to add an ATA hard drive with a capacity greater than 137GB to a PC that's more than three years old, you may run into some configuration difficulties. Older hardware and operating systems don't recognize more than 137GB. If you've installed such a model and can't get Windows to access the entire capacity, you may need to upgrade your drive controller, your BIOS, your operating system, or possibly all three.

To check your BIOS version, watch for the version number that shows on your monitor as your PC boots. If the number doesn't display, download Belarc Advisor 6.1, which will provide you with this information and many other details about your system.

Next, check your PC or motherboard documentation, or the PC/board manufacturer's Web site, to see if your BIOS version supports big hard drives. If it doesn't, look for a BIOS upgrade that adds such support. If no upgrade is available for your BIOS, you can avoid the entire problem by adding an ATA-6 host controller card to an expansion slot of your PC - you can pick them up for around £20.

You'll need Windows XP with Service Pack 1 or Windows 2000 with Service Pack 3 to access more than 137GB on a single drive, since the original releases of XP (both Home and Professional editions) and 2000 won't support big drives. You can download Windows XP Service Pack 1a or Windows 2000 Service Pack 3 from the Microsoft Web site.