Whether you call it a clone, a bootable backup, 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 quickly—in some cases, with only minutes of down time.
Consider this typical scenario: you’ve duplicated your Mac’s internal hard disk onto an external FireWire drive. One day you wake up and find that 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the internal disk, with the exception that files added or changed since you performed the backup will be missing or out of date.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 attempt to repair the internal disk—or if it’s completely dead, simply replace it.
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 incrementally —meaning that after the first time, updating your duplicate to reflect the current state of your hard disk requires only copying the files that are new or different.
Because duplicates are so powerful and useful, I recommend that you make them part of your backup strategy.
Due to the proliferation and seeming simplicity of synchronization utilities, many people use duplicates as their only backup (see the sidebar, “Synchronization Utilities” ). 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 contain only the most recent version of each file. If you suddenly realize you accidentally deleted half of your dissertation or erased your contact database before your most recent duplicate, there’s no way to go back and retrieve an earlier saved version.
• Duplicates quickly go out of date. Duplicating an entire hard disk can take hours. Even while your backup is in progress, files are likely to 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 heartily urge you to duplicate your hard disk on a regular basis, that is only part of a solid backup strategy. You should supplement the duplicates with archives (as I describe in the next section).
( Note: 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-R—and 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.)
Sometimes referred to simply as a backup, an archive contains copies of your files as they appeared at multiple points in time. If you want to see the version of a file that existed on your computer two weeks ago, an archive can deliver that—along with today’s version and the version that existed a month ago.
An archive starts with a complete copy of all the files in one or more folders. The next time the backup runs, your backup software could make another complete copy, but because most of the files probably have not changed in the meantime, that would use up a great deal of space—not to mention taking a long time. So backup programs typically perform an incremental archive. This means that on subsequent runs, the software scans the files in the folders you’ve designated and copies only those files that are new (or newly modified) since the last backup. To be truly useful, archives should also be additive , meaning the backup program adds the new or changed files to the archive without overwriting the files already there. That way, you can retrieve many different versions of a given file, and if you delete it on your hard disk, you can still find it in your archive. Thus, what I refer to as an archive is technically an incremental additive archive .
( Note: Some backup programs use the term archive to describe files that have been copied to removable media of some kind for long-term storage and then deleted from the source volume.)
Archives sometimes make use of a snapshot —a list of all the files in the designated folders at the time a backup runs. Even though a certain file may not be copied (because it hasn’t changed since the last backup), it will appear in the snapshot list. You can easily see what the entire contents of a folder looked like at various arbitrary points in the past, and restore it to any previous state in a single operation.
After the initial full backup, archives usually take comparatively little time to run, making it easy to back up your data once (or even several times) each day. This ensures that your most recent backup is never more than a day old. Because they also offer tremendous insurance against accidental deletion (or change) and file damage, archives are an essential part of a good backup strategy. But archives alone are not an adequate solution. I say this for two main reasons:
• Because of the way archives are stored, they do not represent a complete, intact version of your entire hard disk. Ordinarily, an archive is not bootable (at least, not until after you’ve restored it to a fresh disk). If your main hard drive is completely dead, you won’t be able to do any work at all until you’ve replaced it.
• It often makes sense for an archive to include only data files—not your operating system or applications. But reinstalling Mac OS X and applications from their original CDs or DVDs is a lengthy and cumbersome process that you could avoid (or speed up dramatically) with a duplicate of your hard disk.
Archives protect you against inadvertent changes over time, but only a duplicate can get you up and running again quickly after a major problem. In other words, the best backup strategy includes both duplicates and archives.
That said, you can set up both duplicates and archives in many different ways, depending on the hardware and software you have, the types and sizes of files you typically work with, and other variables. In the second part of this article, I’ll make some general suggestions on strategies you can take.
[ Joe Kissell is the author of several books about Macintosh software, including Take Control of Spam with Apple Mail ( Tidbits Electronic Publishing, 2004) and curator of Interesting Thing of the Day. ]