Almost every Mac with a FireWire port can boot from an external FireWire drive (either the 400 or 800 variety). However, only Intel-based Macs can boot into Mac OS X from USB 2.0 hard drives. Therefore, if you're looking for a drive on which to store duplicates, I suggest choosing either FireWire-only or combination FireWire/USB hard drives, which will give you the broadest compatibility.
However, be aware of two important points regarding Intel Macs:

• Despite Apple's claims to the contrary, Intel Macs can boot from hard disks formatted using the Apple Partition Map (APM) scheme, which has been the norm on PowerPC-based Macs for years. The catch is that currently, the Tiger installer refuses to recognize such disks as a valid destination and instead requires you to reformat the drives with Disk Utility to use the new GUID Partition Table (GPT) scheme. Luckily, however, you don't need to worry about any of this when making backups. If you use one of the utilities described in this article to create a duplicate from your Intel Mac, it will be bootable even if the volume uses APM.
• If you have an Intel Mac with any version of Tiger (10.4) on it and make a duplicate of your startup volume, that volume will not boot a PowerPC-based Mac; likewise, a duplicate of a PowerPC-based Mac's Tiger startup volume will not boot an Intel Mac. Apple is expected to eliminate this inconvenience with the release of Mac OS X 10.5 Leopard, which should be a universal system that will boot Macs with either type of processor.
• Tip
• You can often find bargains on hard drives from a wide range of manufacturers and dealers at dealmac (www.dealmac.com). Drive prices are constantly dropping, and special offers and rebates appear frequently. If you're looking for a particular type or capacity of drive, consider signing up for dealmac's watch listyou'll get an email alert when a deal matching your criteria appears.
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• One additional note: If you end up with more FireWire devices than your Mac has FireWire ports, consider picking up an inexpensive FireWire hub rather than daisy-chaining drives together. A hub gives you the ability to connect or disconnect any drive without affecting the others.

Optical Media

The various flavors of recordable CDs and DVDs are collectively known as optical media, because they rely on lasers to read and write data to them. Most of the Macs made in the past several years include a SuperDrive, which can write to and read from DVD media (4.7 GB) and CD media (up to 800 MB); some have Combo drives that can read from DVDs and write to CDs.
Apple is constantly upgrading the capabilities of the optical drives in their computers. Starting in mid-2005, some Macs included SuperDrives that could read and write to double-layer (8.5 GB) DVD+R discs. In the future, DVD varieties with even greater capacitysuch as Blu-ray and HD-DVDwill become common, and will probably find their way into new Macs.

MacBook Pro models also include SuperDrives that can write to double-layer DVDs. Apple may add this capability to other models in the future.
Because built-in optical drives do not require an additional purchase (except the media, which is relatively inexpensive), it's logical to consider using them for backups. In a few cases they may be adequate, but in general I'd like to steer you away from backing up your Mac onto optical media.
The first thing I should point out is that backing up to any optical media is slow. If you have only a few gigabytes of data to back up, this may not bother you, but as your storage needs increase, you're more likely to find it problematic. True enough, some optical drives are faster than others; a 52x CD burner will obviously require much less of your time than a 2x burner. Even so, the fastest optical drives transfer data at less than one-tenth the speed of the slowest hard drives. And if you're talking about backing up many gigabytes of data, you're still looking at an extremely lengthy process.

Tip
Not sure which kinds of media your Mac's optical drive can record on? Open Terminal (in /Applications/Utilities) and enter:

drutil info

A list of supported media types will appear after the label CD-Write (for CD formats) and DVD-Write (for DVD formats). (In this list, "DL" stands for double-layer.)

Another disadvantage of using your optical drive for backups is that it requires your attention. If your backups run automatically on a schedule, you must make sure a blank disc is in the recorder at the proper time. If you schedule backups for when you're using the Mac (so that you can easily swap discs), you face the possibility that you'll want to use your optical drive for some other reasonand even if not, your Mac may slow down unacceptably during the backup process.
Financial considerations alone make optical media an attractive option, despite their disadvantages. But before you decide on an optical drive as your backup device, consider the following factors.

Recordable CDs

CDs (including CD-R and CD-RW) make a poor choice for duplicating your entire hard drive. The highest-capacity CDs you can buywhich, by the way, may or may not be compatible with your hardware and softwarehold 800 MB. (Standard CDs hold either 650 or 700 MB.) In order to duplicate your entire hard diskeven with the smallest possible installation of Tigeryou would need four to six discs, depending on their capacity. And if you want to duplicate a full 120 GB hard disk, that will require upwards of 170 discs! Even then, you will not be able to boot from your duplicate; you'd need to restore it to a hard disk first. Because of the number of discs required, the amount of user interaction the backup will require, and the inability to boot from the final product, CDs are a bad idea for duplicates.
When it comes to archive backups, CDs show a bit more promise. Yes, it still takes a stack of them, and yes, that means time-consuming sessions of swapping (and labeling!) discs. However, if you're backing up only your data files (not your entire hard disk)and particularly after your first session, when you're incrementally backing up only changed filesthe time and aggravation it requires will be much less. As CDs go, CD-RW media has an edge over CD-R (even though it's almost twice as expensive) in that it can be erased and reused when your stack of discs becomes too large.

Recordable DVDs

Recordable DVDs may all look alike, but they vary in format and capacity. Early Apple SuperDrives supported only DVD-R media, though with the right software, you could also use erasable DVD-RW media. A pair of competing standardsDVD+R and DVD+RWis supported by currently shipping SuperDrives and most third-party external DVD recorders. In addition, newer third-party drivesand SuperDrives in most Macs shipped from mid-2005 oncan use double-layer DVD+R media with a capacity of 8.5 GB (a single-layer DVD can hold up to 4.7 GB).

Note
You will sometimes see drives described as supporting "DVD±R" or "DVD±RW." The ± symbol means both + and (as in, DVD+R and DVD-R). And if a drive supports a rewriteable format, it also supports the corresponding write-only format. So, for example, a DVD±RW drive also supports DVD-R and DVD+R.

Another standard, known as DVD-RAM, is also supported by many third-party drives (as well as some older Macs). Depending on the format, a DVD-RAM disc can hold up to 9.4 GB of data.
Despite these differences, recordable DVDs all share the same basic traits: significantly higher capacities than CDs, offset by much slower recording speeds.
First, the good news: if you want the lowest possible cost per gigabyte of storage over the long run, you can hardly do better than DVD-RW (or DVD+RW) discsif your optical drive and software supports them. Buy a package of 50 (typically sold without cases on a plastic spindle) for under $50, and you have enough media to back up a medium-sized hard disk for a couple of years. When all the discs are full, erase them and start again. DVD-R discs, although not erasable, are a bit cheaper than rewritable DVD-RW or DVD+RW media, and will work with any SuperDrive. DVD+R DL discs hold more data, but are not erasable.
But there's a catchseveral catches, in fact:

• Even the highest-capacity recordable DVDs may not be able to store the entire contents of your hard disk.
• In cases where you can duplicate your entire hard disk onto a DVD, you will still, in general, be unable to boot from the DVD. As with CDs, you must restore the duplicate onto a hard disk first.
• Erasing rewritable DVDs (DVD-RW and DVD+RW) can be rather time-consuming.

Final Thoughts on Optical Drives

I believe the best backup strategy requires the least manual effort. Because optical media tend to require a lot of manual effortand because they do not provide you with a bootable backupthey're less than ideal. However, if you've just spent your entire savings on a new iMac and you can't possibly spring for even a single external hard drive, backing up onto optical media is vastly better than not backing up at all. Just keep these thoughts in mind:

• For minimum inconvenience, use the highest-capacity discs your drive supports (i.e., DVD rather than CD).
• If saving money is paramount, use rewritable media (DVD-RW or DVD+RW), if your drive and software support it.
• Because incremental duplicates are impossible with optical media, plan on making a duplicate just once a month.

Magneto-Optical Disks

Several different manufacturers offer magneto-optical (MO) drives and disks, ranging in capacity from 128 MB to 9.1 GB. Some of these are write-once like CD-Rs (the acronym WORM, for Write Once, Read Many, applies to such disks and drives); others are rewritable like CD-RWs and can be erased. The primary advantage of MO technology over CDs and DVDs is longevity: MO media is typically rated for long-term archival storage on the order of 100 years. On the other hand, MO media is extremely expensive, as are the drives themselves. The mechanisms are considerably slower than conventional optical drives. And MO media comes in many different formats and sizesonce you choose a media type, your future options may be limited.
The latest development in MO is called UDO (Ultra-Density Optical), with disks that can hold as much as 30 GB each. If regular MO drives and media are expensive, UDO is out of this world: plan on spending about $4000 for a low-end drive, plus upwards of $60 for a single rewritable cartridge. Because the backup plan I'm recommending here does not require extremely long-term storage of media, and because I assume you do not wish to spend more on your backup device and media than what you paid for your Mac, I see no reason to consider MO or UDO drives.

Other Removable Media

Besides optical discs and magneto-optical disks, you can find many other removable storage devices, from a wide range of manufacturers. The most popular onesand, for our purposes, the only ones potentially worth consideringare made by Iomega.

Iomega Zip and Jaz

Iomega Zip drives store 100 MB to 750 MB on removable magnetic disk cartridges that are slower than hard drives, but usually faster than optical discs and much faster than tape drives. Although the cost of media per gigabyte is comparatively high, Zip disks can be reused indefinitely. The same is true of the now-discontinued Jaz drives, which support 1 GB and 2 GB Jaz disks. Unfortunately, Zip and Jaz disks have a reputation for being unreliable, so I recommend against using them for backups.

Iomega REV

Iomega's latest removable-storage device, REV, uses rugged, hard disk-based cartridges that hold 35 GB each (www.iomega.com). Designed as a faster and more robust backup platform than tapes or DVDs, the FireWire version of this drive even includes a free copy of Retrospect Express. Although REV is significantly slower than ordinary hard drives, the cost of a single drive (about $300) plus several disks (about $50 each) could be less than the cost of two or more stand-alone hard drives.
The only significant downside is that 35 GB is not large enough to hold the contents of some users' startup volumes; although you can certainly split a backup onto multiple disks, this would prevent you from making a bootable duplicate. However, if you have well under 35 GB of data on your startup volume, REV may be an excellent choice. A word of caution: REV is a recent and relatively untested technology. Given the poor reputation of Zip and Jaz drives, it might be wise to wait for evidence of reliability before trusting your data to REV.

Tape Drives

For many years, digital tape drives were considered the only reasonable, cost-effective way to back up large quantities of data. They're still extremely popular in large businesses. Common digital tape formats include VXA and DDS (a data-optimized variant of DAT, digital audio tape). Although at one time tapes were notorious for losing data spontaneously, they have now achieved a comfortably high level of reliability and longevity. And in (extremely large) quantity, they can be quite economicalthough most of us will never get to the point where that economy of scale kicks in.
Tape drives have many virtues, but speed is not one of themat least, not for the lower-end tape drive most of us mere mortals can afford. It takes far longer to back up a given amount of data to a tape than to even a slow optical disk. Restoring files is even more time-consuming, because tapes must be rewound or fast-forwarded to the correct spot before the data can be transferred. And you will never be able to boot your Mac from a tape drive.
When truly phenomenal quantities of data must be backed up, when money is no object, and when time is plentiful, tape drives are perfect. High-capacity tape librariesautomated systems that can robotically swap tapes into and out of a bank of tape drivesare marvelous (and marvelously expensive) toys that form the backbone of many corporate backup systems. But for ordinary people with modest amounts of data, too little time, and even less money, they make little sense. Consider that you may spend about $1600 for a drive that supports 80 GB tapes, which in turn cost about $80 each. For that price, you could buy sixteen 80 GB hard drives or four 500 GB hard drives, which should be enough to provide speedy, redundant backups for all but the most extreme Mac setups.