If you have more than one Mac but don’t want to use a separate hard drive to back up each one, Time Machine can help.
Time Machine, of course, is the automated back-up feature introduced in Mac OS X 10.5. Time Machine works in the background, creating backups of files on an external hard drive whenever you modify your system.
You can use a single drive to back up multiple computers with Time Machine in any of the follow three ways. Before you do so, make sure you have enough free space on the drive—add up the space currently used on each of your computers and multiply the sum by 1.2 to find the minimum amount of storage space you’ll need.
Method No. 1: SneakerNet
One approach is to simply attach a drive to one Mac, let Time Machine run, and then disconnect the drive and attach it to another Mac. Time Machine stores backups for each computer in a separate folder, so you needn’t worry about one computer’s backups overwriting, or otherwise getting confused with, the other’s.
Before disconnecting a drive, click on the eject icon next to its name in the Finder sidebar. You don’t have to turn off Time Machine, however; it will patiently wait until you reconnect the drive is reconnected to before it runs again.
Moving a drive from one computer to another has the advantage of speed: backups will proceed much more quickly than they would over a network. However, it goes without saying that each computer can add files to its backup only with the drive attached to it.
Method No. 2: Network Backups
The other approach is to leave the drive connected to one of your Macs, share it using Personal File Sharing, and connect to it over the network (either a wired Ethernet network or a wireless AirPort network) that includes your other computers.
Once again, Time Machine is smart enough to keep one computer’s backups separate from another’s, even if it’s using the same disk. But if you’re using Time Machine to back up several computers over a network, you need to keep a few things in mind.
For starters, network backups are generally much slower than backups directly to FireWire or USB drives. Likewise, backups over AirPort networks are slower than backups over Ethernet, and the older AirPort protocols (802.11b and 802.11g) are slower than 802.11n, the most recent standard for AirPort Extreme Base Stations and cards.
Also, when your computers are busy backing up or restoring files over the network, other network activities (such as downloading files or streaming video) can slow down significantly. Finally, network backups require the computer sharing the disk to be turned on and awake. If that computer is turned off or goes to sleep, Time Machine must wait to resume backups until it reappears on the network.
Method No. 3: Time Capsule
The easiest (though perhaps not least expensive) way to back up multiple Macs is to buy a Time Capsule, Apple’s recently announced network backup appliance.
Time Capsule combines the features of an AirPort Extreme base station and a large (500GB or 1TB) hard drive that’s preconfigured for use with Time Machine. After plugging in the device and configuring its network settings (as you would with any AirPort base station), you need only select Time Capsule as the backup destination for Time Machine on each computer in your home or office.
Although Time Capsule supports 802.11n, you’ll see maximum benefit from its high-speed wireless networking only if your Macs also use 802.11n—and even then, remember that wireless backups will proceed less quickly than backups over a wired Ethernet connection.
Another disadvantage to this option: It's not yet available, as of this writing. Time Capsule is slated to ship in February. When the storage device does arrive, Macworld will have a full review.