I’ve run into a new behavior that was a bit puzzling in Macs running OS X 10.5 or 10.6. The symptom is that when the machine starts up, the monitor displays the gray Apple for a couple of seconds, then the Mac shuts off totally.
On investigation, it seems that the hard drive directory would have an unrepairable problem on the index, something that Drive Utility could not fix. In OS X 10.5, and 10.6, the operating system does a file structure check at boot when the system was forced to shut down or suffered a freeze or anything other than a normal shut-down.
This directory structure check takes the form of a progress bar that shows up in 10.5 and 10.6. If this check fails right off the bat, the system shuts down right away to prevent damage to the HD from booting from an inoperable index.
It’s a great technique. The first time I saw the Mac shutting down, I wasn’t sure I understood what was happening, but now it’s confirmed, the OS is doing a preventative shutdown.
