Either the 10.8 disk is damaged & unbootable, or the 10.9 update altered the firmware to make the older OS unbootable.
I doubt it is the latter, so it's probably a good idea to consider repairing the 10.8 disk.
Do you have another Mac to try booting the 10.8 disk from? If you can make a 10.8 USB installer you can boot from that to run 10.8's Disk Utility to repair the 10.8 disk (avoid cross version repairs if at all possible). If that fails to boot you know the Mac is no longer willing to run 10.8.
Bootable USB installer…
You could probably use the regular startup commands to try to troubleshoot the OS (verbose mode & single user mode), but they are tough to use, hit the key combination after selecting the 10.8 disk in the boot picker (when you hold alt at startup).
Verbose is simply a case of tying to see an error message scroll past (record it on video ).
'Blessing' should happen as you use the GUI in system preferences, the act of selecting the disk is meant to bless the relevant bootloaders IIRC.
There is the bless command…
I'm just not sure on the syntax for 10.8 (& I am unconvinced that is the issue here).
Many, Many Thanks for your very informative and comprehensive suggestions for a solution.
Nope I do not have another Mac ... so I need to try the bootable USB intaller thing.
I will go away now (I have an empty 16 GB USB Stick) and will download the already purchased "Mountain Lion Installer" (4.36 GB) from Apple.
Will comeback with the results...
Message was edited by: NULCGI
I was successful in creating a Bootable USB installer (for installaion of 10.8)
Startup from the USB installer went OK and I was able to select/run its version of Disk Utilities
and use that to verify and repair the troublesome 10.8.2 Ext. HDD. Having completed that task
I tried to reboot from the HDD - alas... with no success (blank screen forever remained)
On booting from the USB Installer again, I was able select the troublesome 10.8.2 Ext. HDD and opt
to re-install Mac OSX 10.8 on to it. (hoping that would resolve matters)
On completion of that process, the system then restarts - in order to complete the 10.8 istallation process.
Because the new and re-installed version 10.8 has yet to be completed, the issue of non-booting via the
10.8.2 HDD remains !
So, I have been unable to resolve/fix my problem - what are your thoughts?
at last, I have managed to get an external HDD (Mac OSX 10.7.5) to boot from my iMac (10.9.2 Mavericks)
I only acheived this having found an old sata HDD (Mac OS X 10.6 installed) which I put into a HDD Dock.
Once it booted OK... I was able to upgrade the system to 10.7.5 - without issues.
However, my aims here have been snookered by the fact that the Software I wish to reinstall on the ext. HDD
uses Universal Installer.app which will not run on 10.6 or 10.7 ???
1. How old does the Mac OSX need to be - to accomodate Universal Installers (PPC/Intel) ?
2. Is there a way to modify a Universal Installer - to remove the PPC content ?
All help appreciated to fix this ongoing dilemma