External hard drives are ideal for backing up your hard drive or storing additional media separately from your main hard drive. Photos, videos, and digital music have all become so popular in recent years that people need a place to put it all. CDs and DVDs do not have the capacity to hold PC backups and are not all that convenient. However, the prices of external hard drives have come down so much that they are perfect for backing up your important data.
The two most common interfaces for hooking up external drives to your computer are USB and FireWire with eSata now becoming popular also. USB has been around for years and are standard on just about all computers today while FireWire ports are newer and if you use a digital video camera with your computer you probably already have one. In terms of access speeds the latest eSata standard offers the fastest connection however that comes with a price. USB 2.0 and Firewire can cover most of your general needs without having to upgrade your system with a new eSata card. Any of the interfaces will do the job however be sure to purchase an external hard drive with the same type of port that your computer has.
Your backup drive should have a capacity at least as big as your computer's largest hard drive. That will leave you with plenty of room to back up your important files or mirror/clone your current drive. You can also buy an external hard drive case and mount an internal hard drive inside it. This method will give you a choice of brands and models, and if anything goes wrong with your internal hard drive, you can simply install the external one inside your computer to replace it.
One drawback to using external hard drives for backups is that they are still relatively new for home computer use. They come with Windows drivers; should Windows fail, you will not be able to access your external drive through DOS. There are DOS drivers available but they can be difficult to find and most manufacturers do not provide support for them. This isn't a frequent occurrence, just something to keep in mind as you select your external hard drive.