Most modern hard drives have built-in sensors and monitoring circuitry that form a system called S.M.A.R.T. (Self-Monitoring Analysis and Reporting Technology). The idea of S.M.A.R.T. is to detect the warning signs of potential problems before they occur. Although S.M.A.R.T. cannot detect every possible drive problem, it can provide one very valuable warning: "Your drive is about to have problems, so back it up and repair (or replace) it now!"
What does it work with? As of mid 2006, Disk Utility's S.M.A.R.T. indicator works with internal ATA and Serial ATA drives, but not with external (USB or FireWire) drives. Some external drives, however, have their own built-in S.M.A.R.T. indicators.
To check your drives' S.M.A.R.T. status, launch Disk Utility (find it in /Applications/Utilities) and select a drive in the list on the left. (Be sure to select the drive itself, not one of the indented volumes beneath it.) If the selected drive supports S.M.A.R.T., you should see this at the bottom of the window: "S.M.A.R.T. Status: Verified". If you see "About to Fail" in red letters, back up the drive immediately. You can then use Disk Utility (or a third-party repair utility) to attempt to repair the drive, but more often than not, "About to Fail" indicates an imminent hardware failure that you cannot fix with software. Even if Disk Utility does appear to solve the problem, don't trust the drive with important data; replace it as soon as possible
Tip
To monitor your drives' S.M.A.R.T. status in the background (without having to remember to open Disk Utility), try SMARTReporter, which displays a status icon in your menu bar (http://homepage.mac.com/julianmayer/, free).

Decide on a Backup Strategy

I know a number of people who have made decisions about backing up their computers based on what hardware or software they already own. Others buy a product that's received good reviews and then figure out how to use it for effective backups. I believe these approaches are backward. If your data and your time are truly important, it makes sense to think about your needs first, then develop a strategy based on those needs, and finally choose hardware and software that fits your strategy.
When earlier versions of this manuscript were published, several readers commented that the strategy I suggest here, while perfectly reasonable, may be inappropriate for "low-end" users because it presumes a significant expenditure of money and effort. Less-advanced users, the argument went, just want a backup system that's inexpensive, easy-to-use, and effective. Don't we all! Unfortunately, there is no such thing. You know the old saying: "Cheap; good; fastpick any two." The same goes for backups. I can tell you how to do them effectively or how to do them quickly and cheaply, but the less time and money you're willing to spend, the less safe your data will be.
With that in mind, I want to begin this strategy section with a quick, high-level overview of several approaches you might choose to take, depending on your tolerance for cost, effort, and risk (see Table 1, "Sample Backup Approaches"). Later on, I describe in detail each of the hardware, software, and strategic components of these options.

While the approaches I outline are just a few examples of the many paths one could take to performing backups, I personally feel the importance of protecting your data trumps all other concerns. Therefore, in Table 1, I outlined the Data Safety approach in bold, because I believe it is the best approach for the majority of readers of this article. If your data is not worth some time and money to you, then you probably don't need backups. But if data safety truly matterscan you afford to lose your email, one-of-a-kind digital photos, or important documents?keep in mind that you get out of a backup system what you put into it.

Tip
There's an even more secure level beyond the "Data Safety" option in Table 1, but implementing it takes a bit of doing. Make these modifications to the plan:

• Use hardware-encrypted hard drives (see Choosing a Hard Drive).
• Using SoftRAID, partition each of the external drives into a volume for archives and a volume for duplicates (see Can a RAID Substitute for Duplicates?.
• Rotate the drives more frequently (say, once every two or three days) and keep one or more of them offsite at all times.

Do You Need Duplicates?

Let's begin by assuming you have original (CD-ROM or DVD-ROM) copies of your operating system and all installed software. Now consider this question:
If your hard drive suffered a complete failure, how much time could you afford to spend restoring it to working order?
If you use your computer to run a business, do your homework, or trade stocks, for example, your answer may be "a few minutes at the most." If no critical projects depend on a functional computer, you may be able to afford several days to restore it after a failure. Most of us are somewhere in between.
In the best case, it will take you several hoursand possibly a day or moreto reinstall a typical set of software onto a new or reformatted disk. However, if you do not have original copies of all your software, if you have a large number of third-party applications, or if you've customized your computer extensively, returning your computer to operation could take much longer.
The more you need to avoid that potential loss of time, the more you need to maintain duplicates (for more info, see The Duplicate).

Do You Need Archives?

Regardless of your need for duplicates, consider your answer to this much different question:
If your computer were stolen, how difficult would it be for you to live without the data on it?
Do you have years of bank records, email, poetry, academic papers, photos, movies, and so on stored on your computer? If so, chances are your answer is "extremely difficult." On the other hand, if you use your computer only for casual Web surfing, playing games, and listening to music, living without the data on your computer may be little more than a minor inconvenience.
Although a duplicate includes a copy of your data, an archive includes many different versions of your data, making it much more likely that you'll be able to retrieve the information you need in the event of a problem.

The greater the amount of personal data on your computerand its importance to youthe greater your need to maintain archives.
Though there may be some exceptions, the ideal backup strategy for most people consists of both duplicates and archives. I discuss each of these in the pages that follow.

Do You Have Special Backup Needs?

Although duplicates and archives cover most situations the typical user will encounter, some people have special backup needs that don't quite fit the mold. I'm thinking, in particular, of users with large numbers of digital photos and those who work extensively with the large files required for digital video or pro audio applications.

Digital Photos

Many people, when asked what one item they would try to save if their house were burning down, would answer "my photo album"because furniture can be replaced, but memories cannot. The same thing is true of the memories stored on your hard disk in the form of pictures you've taken with your digital camera.
Most of us have at least a few digital photos on our computers. But some people take pictures constantly, and feel justifiably concerned about entrusting this irreplaceable data to their computers. Also, digital camera resolution is constantly on the risemeaning the next new camera you buy is going to require more space for the same number of images as your previous one. Your new mobile phone probably has a camera, too. As the number and size of your images increases, you may find that duplicates and archives alone don't entirely meet your backup needs.
For one thing, it can be extraordinarily difficult to find just the right photo from among thousands of similarly named files when it comes time to restore your data from a backup. Although Spotlight can use keywords and other metadata to help you find photos when they're on your hard disk, it won't help you when they're on a stack of DVDs. (For solutions to this problem, see Cataloging software)
Photos are also among the files you're most likely to share with other people. If you've ever created an online photo album using iPhoto, iWeb, or .Mac HomePage, you know how easy (and addictive) photo sharing can be. Although the files you've shared on the Web do, in a sense, constitute a backup of the ones on your computer, you probably haven't shared all your files onlineand you most likely uploaded low-resolution copies of the images anyway. Wouldn't it be great if you could back up all your photos online, and still have the ability to share just the ones you want? (You can! I explain how in Photo-sharing services)
Finally, let's not forget that photos are especially valuable. Although you wouldn't enjoy spending months rewriting The Great American Novel, it's at least possible. Recreating photos of a new baby or an important life event, on the other hand, simply can't be done.
Luckily, numerous tools, services, and strategies exist for the express purpose of making photo backups as painless and secure as possible. To learn more about them, read Photo Backup Strategy.

Video and Audio

Video files consume an enormous amount of disk space, and when you're working on editing a large video project or producing DVDs, the file sizes can become truly staggering. Add HD video content to the mix, and the file sizes balloon even further. Because of the sheer quantity of data you may generate, conventional duplicates and archives may not make the most sense. You're also likely to create numerous intermediate files between the raw footage and the final product, and deciding whether or how to back up that data can be challenging.
All this is equally true for those working with audio production, especially when your Mac functions as a multitrack recorder; it also holds for photographers working with gigantic, ultra-high-resolution images and several other categories of user.
So ask yourself this question:
Do you frequently generate more than a few gigabytes of new or modified files in a single day?
If you're working with large video, audio, or still image files, the answer is likely yes. All that data can strain conventional backup methods, not to mention your pocketbook. Learn more about how to get the job done without breaking the bank in Video and Audio Backup Strategy.

The Duplicate

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.

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. 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).

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-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.

The Archive

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 2 weeks ago, an archive can deliver thatalong 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 spacenot 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 additive incremental archive.

Synchronization Utilities

Lots of utilitiesincluding several that bill themselves as backup toolsperform synchronization. As the name implies, synchronization means maintaining identical copies of a file, folder, or even an entire disk in two or more locations. Some synchronization utilities can run on a schedule, automatically "backing up" files from a location you specify to another volume. And some can create a bootable duplicate by synchronizing an entire disk to another disk.
There's nothing wrong with synchronizationin fact, it can be incredibly useful in certain circumstances, such as keeping your laptop's hard disk updated with documents you use frequently on your desktop Mac. As a quick and easy way of making an extra copy of certain files, it can serve as a type of primitive backup.
If you want to use a synchronization utility to make duplicates as part of your backup strategy, that is perfectly valid too. However, please do not mistake synchronization for a true backupno matter what the utility's advertising says.
What's true of duplicates is equally true of individually synchronized files and folders: you get only the most recently modified version. You lack the ability to recover an older version of the file, which is a crucial part of a solid backup program. Also, if you don't notice that a file is damaged before synchronizing it to another volume, you may end up with two useless copies. If you synchronize deletions, you lose your insurance against accidentally trashing files. And it's all too easy to accidentally copy data in the wrong direction!
All that to say: a single copy of a single version of your data does not a backup make. By all means, synchronize if you wish, but not as a substitute for proper archives and complete, bootable duplicates.
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 filesnot 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.

Scheduling Backups

I can say from personal experience that backups are far more likely to happen regularly if your backup software runs automatically on a schedule. And let me be quite clear: regular backups are the only kind that matter. I think it's fair to state this as a corollary to Murphy's Law: "The likelihood of suffering data loss increases in direct proportion to the elapsed time since your last backup." In other words, if you're performing all your backups manually, the one day you forget (or run out of time) will be the day something goes wrong.