Even if your backup application reported no errors, you should test the duplicate to make sure it truly is bootable. If your duplicate was stored directly on another hard disk, testing it is easy. (If it was stored on optical media, see Restore a CD/DVD Duplicate onto a Hard Disk, page 178.) Follow these steps:
Open System Preferences and click the Startup Disk icon.
Select the volume where your duplicate is stored. (You did give it a unique name, right?)
If you duplicated your hard disk to an external drive connected to a server, you must physically connect that drive directly to the Mac you want to start up. If it's on another machine, it will not appear in the Startup Disk preference pane. The only way to boot a Mac over a network is to use NetBoot to load a special disk image stored on a central machine running Mac OS X Server; an ordinary hard drive won't work, even if it contains a bootable copy of Mac OS X.
After your computer restarts, verify that it used your duplicate as the startup volume. If your Finder preferences are set to display mounted hard disks on the Desktop, the one shown at the top is your startup volume. (To set this preference, choose Finder > Preferences, click the General icon, and make sure the Hard Disks checkbox is selected.)
If your computer did not start from the correct volume, restart it again, holding down the Option key until the screen displays icons for each of the valid startup volumes. Click the volume you wish to use and then click the right arrow button to complete the startup process.
If your computer refuses to boot from a FireWire drive even after holding down the Option key at restart, one possible cause is a conflict with other FireWire devices (such as an iSight camera). Disconnect all other FireWire devices from your system and try again with only your external hard drive attached.
Do a few spot checks to confirm that important files are where they should be, that you have network access (try viewing a Web page), and that a few applications launch. I recommend not checking your email, though, as doing so may download messages and delete the originals from the serveryou'll miss them when you return to your usual startup disk.
Return to System Preferences, click the Startup Disk icon, choose your usual startup disk, and click Restart.
You've just confirmed that your duplicate works correctly. If your computer does not restart from your duplicate volume, however, your backup software may have malfunctioned. Try performing the duplication again, consult your software's documentation, or contact the developer's technical support department for assistance.
Using an External Drive as a Startup Volume. All modern Macs (those manufactured since approximately 2000) can boot from an external FireWire hard drive, assuming the drive was manufactured to the proper specifications; Intel-based Macs can also boot from USB 2.0 drives. See the sidebar USB 2.0 Drives, Intel Macs, and Bootability (page 121), for more information.
If you have trouble booting from an external drive, check Apple's Web site to confirm that your machine supports booting from the interface you're using. Also check the drive manufacturer's site to see whether any firmware updates are available for your drive.