External hard drives are a popular choice for bulking up a Mac's storage capacity. Externals can be used for many purposes, including data backups, storing large files (such as your iTunes library), or storing the thousands of family photos you've accumulated over the years.
It's important to select the right type of external drive, both to ensure that it's compatible with your computer and to ensure that it will meet your needs for the foreseeable future. Buy the largest hard drive you can afford, even if at the moment you can't imagine ever needing that much space. You may be surprised at how quickly it fills up.
External Drive Types
External hard drives come in two basic styles: an empty case, and a case that includes a pre-installed hard drive. The pre-installed version is the most popular, and is easy to find at computer-related stores.
Another factor that determines external drive type is how it connects to your Mac. External drives usually use one of the following methods: USB 2.0, FireWire 400, FireWire 800, or External SATA, also referred to as eSATA.
Note: A new version of this article is available that includes information on recent innovations in connection and drive technology: Increase Storage With an External Drive for Your Mac
Empty External Drive Case
Though not as popular as pre-installed hard drives, this type of external drive case is worth considering. It's essentially a complete hard drive enclosure, with a power supply and hard drive interface; about all that's missing is the hard drive itself.
If you buy an external hard drive enclosure, you can install the drive yourself, whether it's a new drive or an existing drive that you swipe from an older computer.
Installing a hard drive in an external case is a task that almost anyone can perform. You don't need to be a computer guru, just handy with a screwdriver.
Pre-installed External Hard Drive Enclosures
This is by far the most common form of external hard drive. The enclosure contains a hard drive, power supply, and hard drive interface; all you need to do is plug it into your Mac.
Pre-installed external hard drive enclosures are available from leading hard drive manufacturers, as well as some of the well-known names in Mac peripherals. When selecting one of these drives, the main considerations are storage size and connection type.
Connection Type: USB
Most external hard drive enclosures support USB 2.0 connections. Macs as far back as the original candy-colored iMacs support USB-connected external hard drives.
One of the drawbacks of USB 2.0 is that PowerPC-based Macs can use these drives, but not boot from them. Intel-based Macs, however, can boot from USB devices. This is only a concern if you plan to use the external hard drive for backup purposes; even then, the ability to boot from the drive is more a convenience than a necessity.
Connection Type: FireWire
Though not quite as common as USB, FireWire is one of the preferred connection methods for Macs, because nearly all Macs have built-in FireWire support. Any Mac that supports FireWire can boot from an external FireWire hard drive enclosure. This makes external FireWire drives an almost universal choice, because they work with both PowerPC and Intel-based Macs.
FireWire external drives are available in two formats: FireWire 400 and FireWire 800. FireWire 800 is the newer and faster of the two, although either will work with any Mac that supports FireWire.
Connection Type: eSATA
eSATA is a relatively new connection method. Currently, no Mac directly supports this type of connection without an adapter, such as an eSATA PCI-E card for the Mac Pro, or an ExpressCard for a MacBook Pro.Nevertheless, eSATA is becoming more popular, and I wouldn't be surprised to see Macs directly support this connection method in the near future. In the meantime, many Mac Pro users are choosing eSATA drives for external storage, due to their relatively low price and the ability to use the same SATA drive either internally in a Mac Pro or externally in a hard drive enclosure.
Enclosure size actually refers to the size of the hard drive that an enclosure supports. Two prominent sizes are available: 3.5" is the standard size for hard drives in desktop computers; 2.5" is the standard size for hard drives in laptop computers.
Unless you're re-purposing a laptop drive, I don't recommend 2.5" external enclosures. Generally speaking, these drives are slower and store less than their 3.5" cousins.
So, What Should I Get?
My universal recommendation is an external hard drive enclosure that supports both USB and FireWire. This configuration is fairly common, and allows you to connect the enclosure to your Mac using either method. This gives you more bang for your buck, because you can use the drive with any Mac you have now or any Mac you buy in the near future.
As far as size, I recommend buying the largest drive you can comfortably afford. I also recommend going one or two sizes smaller than the largest drive that's currently available, because that's usually the sweet spot in price.