I own a Western Digital My Passport Studio 320 GB external hard drive, which I purchased a few weeks ago as a backup drive for my MacBook Pro. The drive came with three cables, a FireWire 800 cable, a FireWire 400 cable, and a USB cable. The drive obtains its power through the cable from the FireWire or USB bus. CNET has a review of an older model of this drive.
I used Carbon Copy Cloner to create a complete backup of my MacBook Pro to the external drive. Unlike Apple’s Time Machine, Carbon Copy Cloner creates a bootable backup. I checked that I could boot my Mac from the external drive by restarting while holding down the option key. However, my external drive did not appear in the list of bootable drives. Perplexed, I performed a google search to see if other people had experienced this problem, and I found this solution at macosxhints.com.
The solution is to shut down my MacBook Pro and unplug the external drive, then boot while holding down the option key. Once the Mac presents the list of available boot drives, I plug in my external drive, whereupon the Mac adds the external drive to the list of boot drives. I use the arrow keys to select the drive and press the return key to boot from that drive.
After booting, I verified that my external drive had been used as the boot drive by checking that special icons were used for my System, Applications, and other folders. (Generic folder icons are used for these folders when they are present on a drive that was not the boot drive.)