Have you had an OS X update go bad on your Mac? What did you do? Macworld’s Rob Griffiths had an update that died, leaving his iMac dead in the water; stuck CD, and unable to start up:
Finally, I remembered the old standby, FireWire Target Disk Mode… I have no idea what went wrong the first time, but I’m very thankful FWTDM was there to save me the hassle of reinstalling from scratch.
Yet another good reason to have FireWire on a Mac, though Intel-Macs will start up with an external USB drive. Two words: ‘Back up.’