Features

EXTERNAL CONNECTIONS

With the exception of a few high-end solutions using eSATA or SCSI for ultra-fast data transfer, most external hard disks have USB 2 and/or FireWire 400 connections. Many of the more professional models for both 2.5in and 3.5in disks can also feature the faster FireWire 800 connection.

In theory, USB 2, with a theoretical maximum throughput of 480Mbit/sec, should be slightly faster than FireWire 400, at up to 400Mbit/sec. However, in reality, the same drive connected over USB 2 performs significantly less well than when it's connected over FireWire 400.

FireWire 800 ports are much less common: in the Mac range, they're only found in Power Mac G5s and high-end PowerBooks and MacBook Pros, and in an even smaller percentage of the PC desktop and notebook market. Again, although the theoretical transfer rate is greater than the previous generation - 800Mbit/sec versus 400Mbit/sec - you don't get double the data-transfer speed. The controllers and physical mechanisms of the hard disks in today's models don't sustain transfer speeds great enough to take full advantage of this faster connection interface.

Older Macs are likely only to include USB 1.1 and FireWire 400, so if you're buying a hard disk in order to transfer data between them and PCs (which tend only to feature USB 2 ports), it makes sense to buy a disk that features both USB 2 and FireWire 400 connections. This means you can connect the drive to your Mac using FireWire to transfer data and then use the faster USB 2 connection when hooked up to a PC.

Limiting yourself

INTERNAL CONNECTIONS

Most modern hard disks use one of two connection types to link into your Mac internally. ATA (Advanced Technology Attachment) is still the more common type of connection, but it's being replaced with the simpler, faster SATA (Serial ATA) technology.

With the older ATA connection, the theoretical maximum data throughput is 133MB/sec, compared with 150MB/sec with the first-generation SATA connection, and 300MB/sec with SATA II. However, speed isn't the only differentiator. (Besides, even today's drives struggle to produce a sustained transfer rate of over 80MB/sec, so these faster theoretical throughputs are, at the moment, only future proofing for the interface). SATA drives have their own dedicated connection and bandwidth, dropping the need to designate one drive as master and another as slave on a single bus. SATA drives can also be hot-swapped.

Except for vintage Macs, pre-G5 towers used ATA drives, as did all laptops prior to the introduction of the MacBook Pro. You can check which interfaces your Mac has by launching System Profiler.

Tower-based Macs can accept a SATA controller card into a free PCI slot to give you access to internal SATA disks, or you could consider adding a card that gives access to external SATA (eSATA) disks, of which the majority currently available are external Raid solutions. eSATA cards are even available for the MacBook Pro's ExpressCard slot (see www.vydeo.com for more information).

CAPACITY

Broadly, you should try to buy the biggest drive you can possibly afford. At worst, you should have around 10% of your hard disk free for the Mac to use as a scratch disk as it performs routine memory management and other tasks. Keep an eye on resellers such as www.ebuyer.com and bigpockets.co.uk for end-of-line and other deals on high-capacity hard disks, and check price comparison sites such as www.pricerunner.co.uk, www.kelkoo.co.uk and froogle.google.co.uk to find the best price.