Whether you call it a clone, a bootable backup, a mirror, or a carbon copy, a duplicate is a complete, exact copy of your entire hard disk that (if it's stored on, or restored onto, a hard disk) you can use to start up your computer if necessary. Duplicates are wonderful because they enable you to get back up and running extremely quicklyin some cases, with only minutes of down time.
Consider this typical scenario: you've duplicated your Mac's internal hard disk onto a FireWire drive. One day your computer won't start at all; the screen displays a blinking question mark indicating that it can't find a valid system. You suspect a catastrophic hard disk crash. No problem: you quickly hook up your backup drive and boot from that. Your computer will behave exactly as if it were running from the internal disk, with the exception that files added or changed since you performed the backup will be missing or out of date. You can then repair the internal diskor if it's completely dead, simply replace it.
Can a RAID Substitute for Duplicates?
RAID stands for Redundant Array of Independent (or Inexpensive) Disks; it's a way of combining multiple physical hard drives into a single logical volume using either software or a special hardware controller. One way to configure a RAID, known as mirroring, is to have the same data written simultaneously to two or more drives. If any one drive fails, another can take over instantly and seamlessly with no loss of data and no down time; you can then replace the faulty drive at your leisure.
I have nothing against RAIDs, and if you need to keep a mission-critical computer running without any hiccups at all, a mirrored RAID might be just what you need. However, I strongly believe that a RAID, by itself, is no substitute for multiple duplicates as described in this book. The best feature of a mirrored RAID is also its Achilles' heel: because changes are reflected on all drives simultaneously, an accidentally deleted file will be immediately deleted on your "backup" drives too! (Stand-alone duplicatesespecially if you maintain two or three of themreduce this risk greatly.) RAIDs address the problem of spontaneous drive failures, but they provide no insurance against human error, theft, natural disaster, or any of the other catastrophes that make backups so important.
That said, you can have your cake and eat it too (for a price). If you use SoftRAID (www.softraid.com; $129), you can create a RAID in which your internal hard disk is mirrored onto two or more external drives at once. You can then periodically rotate one of the drives off-site, where it will function as a stand-alone duplicate of your hard disk at an earlier state. When you plug it back into your computer, it will automatically synchronize itself with the remaining drives in the RAID. The beauty of this approach is that you never have to set up, schedule, or run backup software to make duplicatesit just happens automatically.
This scheme can even be expanded to include archives. Using SoftRAID, it is possible (though awkward) to partition an external drive in such a way that one partition can be used along with your internal drive to form a mirrored RAID while another, non-RAID partition on the external can hold archives. Set up two external drives this way and you're in businessas close to a painless backup system as I can imagine.
You might think it would take a while to make a copy of your entire hard disk, and you'd be right. But most software capable of making a bootable duplicate can also duplicate incrementallymeaning that after the first time, updating your duplicate to reflect the current state of your hard disk requires only copying files that are new or different. Because duplicates are so powerful and useful, I recommend that you make them part of your backup strategy.
However, due to the proliferation and simplicity of synchronization utilities, many people use duplicates as their only backup (see the sidebar Synchronization Utilities, page 94). This is a bad idea. Here's why:
Duplicates provide no insurance against damaged or accidentally deleted files. If your hard disk is missing files, or contains damaged files, when you perform the duplication, those problems will appear in the duplicate as well.
Duplicates quickly go out of date. Even while your backup is in progress, files may change. So if your only backup is a duplicate, you may increase your risk that backed-up files will not be current.
For these reasons, although I urge you to duplicate your hard disk regularly, you should supplement the duplicates with archives (as I describe in The Archive, page 94).
An extra hard drive is certainly the best way to make a duplicate, but you can also duplicate a volume onto a disk image, which can be stored on removable media such as CD-R or DVD-Rand then restored onto a hard drive when needed. By the way, it is possible, though not easy, to make a bootable Mac OS X CD or DVD. Because this process goes far beyond normal backups, I do not cover it here.