Troubleshooting the Booter

Issues at the booter stage are indicated by a flashing dark gray prohibitory icon—evidence of a failure to load the kernel.
To troubleshoot the booter:

>If you’re starting up the Mac from a volume containing a system this Mac has never booted from, the prohibitory icon usually indicates that the version of Mac OS X on the volume is not compatible with your Mac’s hardware.

>Start up the Mac while holding down the Shift key to initiate a Safe Boot. The booter will first attempt to verify and repair the startup volume, indicated by a dark gray progress bar across the bottom of the main display. If repairs were necessary, the Mac will automatically restart before continuing. If this happens, continue to hold down the Shift key. The booter will verify the startup volume again, and if the vol- ume appears to be working properly, the booter will attempt to load the kernel and essential KEXTs again. The booter uses the most judicial, and slowest, process to load these items. If successful, the booter will pass off the system to the kernel, which will continue to safe-boot.

>If the booter cannot find or load a valid kernel, you may need to reinstall Mac OS X on that volume.