Yesterday I encountered something I thought I'd never see: Mac OS X silently failing to boot. Specifically, after installing the Airport updater, my Mac would get as far as the full screen Apple then just sit silently never getting as far as the circular progress bar that should appear underneath. Prior to halting the onscreen Apple would jump downward about half a centimetre. Suspecting hardware failure, I tried booting from the first bootable DVD that came to hand - in this case the 10.4 installation DVD. That worked fine and the mini-OS X on it functioned perfectly. I used the Disk Utility on that to check my hard disk for errors and it reported none. I used it to repair permissions and it found only one mistaken permission and fixed it. Still my HD based OS X would not boot. So I used the startup disk selector on the DVD to try booting into OS 9.2, which my [Powerbook 667Mhz G4] Mac is sufficiently old to run natively. That worked perfectly too, so I concluded that the hard disk was not at fault. I also took the opportunity to back up some stuff that otherwise wasn't backed up to a firewire hard disk. Now I decided to try reinstalling OS X from the install DVD. My first attempt was an "upgrade" installation, hoping to keep all my data files where they already were. The only change after that was that the boot Apple no longer moved down half a centimetre. Still no circular progress bar though. I did note through the log Window that the installer was declining to overwrite files for which I already had a newer version, so I wasn't really hopeful anyway. My last ditch attempt was to allow OS X to do a wipe & reinstall, clearing all old files. That seems to have worked, and I'm writing this from a virgin install of OS X v10.4. Running software updater seems not to suggest an Airport Updater today, so I'm happy about that. My question is - is this story credible? Should I be blaming the Airport Updater, or is that a red herring? I'd also run out of hard disk space midway through the 10.4.4 update a week or so ago, leading to a warning that the install hadn't been completed but the system had continued booting without complaint, told me I was on 10.4.4 in the "About this Mac" dialogue and declined running the update again. Could that be related? Those are the only two recent acts I can think of that took my super-user password. Has anyone ever seen anything similar occur? And was there a less destructive way I could have fixed my OS X? I considered trashing /System from OS 9, but decided that since I really don't know what I'm doing I'd probably better not try anything "clever".