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Your Mac's hard disk is one of its least celebrated but most important components. Without it, not only would you have nowhere to store the documents you create, but the operating system that makes a Mac unique would have nowhere to load from. The only time most of us think about hard disks is when we begin to run out of space, but whether you're replacing an internal drive or looking to buy an external hard disk, there's a lot to consider. In this guide, we'll explain the different physical formats for today's hard disks, making sure you know the difference between SATA and ATA connections, and which of these you need. We'll also demonstrate various configuration options for tower-based systems, provide some general advice for working with data, and make sure you're aware of some Macs' limitations in dealing with hard disks. Along the way, we'll be looking at how to fit hard disks, examine some of the alternatives and discuss when to use the various formatting options offered by Mac OS X.

Remember that a hard disk is used for more than just storing files: it holds your operating system and acts as a temporary dump for files that your Mac creates as it's working. Virtual memory technologies swap out data from your RAM to the hard disk when you're trying to run lots of applications at once, and also provide better performance.

Choosing the best hard disk for your Mac is about much more than just walking into a shop and picking up the highest-capacity option from the shelf. You need to understand

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what you'll be using it for, where it will be used, and if there's a better alternative to your initial plan. That's what we're here to help you with. The flow chart on these pages should give you a better idea of the type of disk you should buy, and our advice over the page will ensure you have all the knowledge you need to make the right choice.

INTERNAL/EXTERNAL

The most obvious choice to make when buying a new hard disk is whether to fit one inside your Mac - possibly as a replacement for, or in addition to your existing hard disk - or buy an external drive that can be added to your setup just by plugging it into your Mac's USB or FireWire connection.

Internal drives are usually cheaper than their external counterparts, simply because external drives charge you for the case and controller technologies, as well as for the basic hard disk inside.

If you want an easily portable disk, you should opt for one based not on the relatively chunky 3.5in mechanisms found in desktop and large-capacity external hard disks, but the smaller 2.5in hard disks. These are not only smaller and lighter, but since they use less power than their bulkier cousins, they can usually draw power from the USB or FireWire buses. This also means you don't have to lug a power supply and cables around with you.

There are portable disks based on even smaller, 1.8in platters, but these are relatively poor value in terms of price per gigabyte when compared with other formats, and they're not substantially smaller.

All external disks basically wrap a casing around an off-the-shelf hard disk, and it's usually very easy to open external cases and swap drives around - although it may void any warranty you have. If you're swapping out one internal drive for another just because you need more capacity, you should consider buying an external case for the old internal one, extending its life and giving you extra flexibility. Cases are available for 2.5in and 3.5in mechanisms. Make sure you pick one with the correct internal connection - ATA or SATA - for the drive you'll be fitting.