A sound backup strategy always includes backups of your backups! Picture this: You've diligently backed up your computer's internal hard disk to an external drive. Then one day, lightning strikes and both drives are damaged. So much for your backup. Even under ordinary circumstances, backup media can fail for all the same reasons your hard drive can fail. So having just one backup, in my opinion, is never enough. You should alternate between two or more sets of backup media for greater safety. If you've set up your backups to run on a schedule, this might mean using set A (a hard drive or a stack of CDs) every day for a week, then switching to set B (a different drive or stack of CDs) for each day of the following week, then switching back—and so on.

So are two sets enough? It depends. Most experts recommend using at least three sets, of which one is always stored off-site. But the three-sets-of-media advice was first given in the days when the media commonly used for backups was much less reliable than what's available today. And the cost of three sets of media—especially hard drives—can be hard to swallow for the average home or small-business Mac user.

In my opinion, except for mission-critical business use, two sets each of duplicates and archives should be adequate for most users. If you back up to hard drives, this can mean two drives, each of which is partitioned to store both a duplicate and an archive (see Partition Hard Disks). Of course, if you can afford a third set, your data will be somewhat safer—and your backup routine will be somewhat easier. But it's better to have fewer sets that you maintain diligently than to lay out an elaborate plan for multiple sets that you don't follow because it's too complicated or time-consuming. In any case, if you have more than one set of media, you certainly should keep one in another location all the time. That brings us to the next crucial part of a good backup strategy: off-site backups.