It's a troubleshooting nightmare: you have problems starting your standard Mac OS X installation, or another serious issue that requires re-installation of Mac OS X itself. You grab your Mac OS X CD or DVD, insert it, press the reset button, hold down the "C" key and expect to be back up and running -- but the system still refuses to properly boot.
This obviously makes re-installation of Mac OS X impossible, and severely limits your troubleshooting avenues. Fortunately, there are some relatively straightforward methods for dealing with this issue.
Try holding down the "option" key instead of the "C" key For some reason, holding down the "C" key at startup does not always work to force booting from media that is in your Mac's optical drive.
In these cases, holding down the "option" key at startup (which, when functioning normally, displays all available startup devices) may allow you to select the appropriate boot volume (your Mac OS X installation CD or DVD) and proceed with the normal booting process.
Failing this, try holding down the "Command", "Option", "Shift", and "Delete" keys simultaneously while your Mac is starting up. This should force the system to bypass the normal startup volume and look for another valid boot drive -- namely your Mac OS X startup CD or DVD.
Your Mac OS X CD or DVD might not be compatible with your Mac If you are using a Mac OS X CD or DVD that came with a Mac other than your own, it may not be able to properly startup your machine. This can occur, for instance, if you purchased a retail copy of Mac OS X older than the version that is required to boot your Mac.
As an example, the first-generation PowerBook 1.5 GHz 17" requires, as a minimum, OS X 10.3.3 to boot. It will not startup from a Mac OS X 10.3.0 installation CD. Other examples: The original 2003 Power Mac G5s require Mac OS 10.2.7 or higher and the 2004 Power Mac G5s require 10.3.4 or higher.
In these cases, simply use the "Restore" CD/DVD that was included with with your Mac, or make sure the retail copy you Mac OS X installation disc meets the minimum requirement for your Mac (the Mac OS X version number should be listed on the CD/DVD label).
Note that this issue also affects third-party startup products. For instance, the CD included with Norton Utilities includes a version of Mac OS X that is too old for most new Macs, and the version of Mac OS X included with some older releases of Alsoft's DiskWarrior is also out-of-date for some systems.
Make sure your firmware is current In some cases, out-of-date firmware can result in an inability to properly startup from optical media.
Apple maintains a chart of available firmware updates for various Mac models in Knowledge Base article #86117.
Go to the aforementioned Knowledge Base article, and download the firmware updater that appears next your Mac's name (if one is listed) and launch the downloaded installer. If your Mac requires the firmware update, the installation process will proceed. If your Mac is already up-to-date, the firmware installer will alert you and no installation will take place.
Re-select the CD/DVD in System Preferences In some cases, simply re-selecting the appropriate startup CD/DVD in the "Startup Disk" pane of System Preferences will allow the system to boot from that media when the Mac is restarted.
Clean your media It may seem like common sense, but dirty optical media has been the cause of failed startups for a number of users. Use a soft cloth (if there is liquid or otherwise sticky residue, slightly dampen the cloth with plain water) to gently wipe the optical side of the startup media.
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