HELP! iMac won't start up - launch_msg(): Socket is not connected

Question

I've got an older 1.8GHz Intel iMac that I run as a server. The hard drive is partitioned into two volumes - one with 10.6.8 on it, and the other with Server 10.6.8. It was running fine until I restarted yesterday - I got the apple logo and the spinning progress wheel as though it was starting fine, but it just stayed like that forever. No problem, I'll just start up from the other partition, right? Wrong - same exact problem, no matter which partition I try to start from. When booting in safe mode verbose, the last message I see is "launch_msg(): Socket is not connected". I run disc utility and it finds no problems. Repair permissions finds some files in apple's help documentation that need fixing, which is done, but the problem persists.
I try to install a fresh system, but the system installer I have is OS 10.6.3, and it won't let me install over the "newer" 10.6.8 system. Argh!
I have an external USB hard drive, so I install a fresh system on that and am able to boot from it just fine. I rename the system and library folders on the internal drive so it won't recognize there's a system installed there. Then I'm able to do a fresh install of Server 10.6.3 on the internal drive. Installation goes fine, but guess what - when it reboots, the same problem still exists! Grey screen, apple logo, spinning progress wheel forever.
I'm about at wit's end here and would love some help!
The signs seem to be pointing to the hard drive being a problem, but disk utility finds no issues, and I'm able to copy files from it just fine. This is a 3TB hard drive that's less than a year old and was functioning just fine. I'm hoping to find a solution that doesn't involve reformatting this beast.
Any ideas appreciated! TIA.

Answer 1

It would be useful to know if you have a good backup of the important stuff on your HD. If not, its time to make one first. You should be able to do this using FW Target disk mode as suggested in my earlier post.
Deleting the iDvd plist file won't have been the cause of the problem, Rhthym.
Rather it sounds as if the computer is starting up in "single user" mode for some reason. (see http://docs.info.apple.com/article.html?artnum=106388 )
First try typing "exit" and hitting return, instead of "reboot".
If this doesn't fix it, I'd reset the PRAM (hold down the command option p r keys while starting up - not just restarting - the computer and wait for the chimes to sound three times ) - see http://docs.info.apple.com/article.html?artnum=2238
The fact that the computer gets stuck at this point, and the password message, also hints at the possibility that a corrupted "Netinfo database" may be the problem. Netinfo is essentially the system that OSX uses to take you beyond the single user "unix" system that you are currently booting into. Replacing this involves playing around with some significant unix commands though, so its not recommended unless you feel confident with such things. You can find instructions on how to replace it at http://docs.info.apple.com/article.html?artnum=107210
If you still can't start up normally you should consider doing an "archive and install" from the OS CD. It may be that another OS system file related to the boot process has become corrupted. Doing an archive and install will leave your data correct , but replace the OS. You will need to re-install any OS updates again though - see http://docs.info.apple.com/article.html?artnum=301270
If you contact Apple technical support tell them that the problem is that your computer is continually booting into "Single User mode".
Cheers
Rod