Well, it happened again! (See
this post for original issue).
Once again, my Mac Pro won't boot from the main hard drive but WILL boot from the Sandbox created from that drive. Same symptom: I hear the chimes, the gray screen appears, the gear spins for about 2 seconds, and the machine shuts down.
Here's what I noticed....after restoring from a month old SD! backup and reapplying changes to the main drive I made in the last month, I again created a Sandbox. I booted off of the Sandbox. A few days later I applied the Combo Updater to update the Sandbox from 10.5.2 to 10.5.4 and then rebooted off of the Sandbox and all seemed good.
Then, a few days after that I wanted to boot off of the underlying main drive, and wham, the Mac Pro shut down, just as it did weeks earlier!
So, it seems there is some evil interaction going on.
Fortunately, I have a relatively recent SD! clone that I made right after getting my HD back to speed (a good caution, I guess!).
I'm going to conduct a test....after creating a new Sandbox, I will (a) make sure I can boot both from the Sandbox and the main drive, (b) apply the 10.5.4 update to the Sandbox, and see if I can boot from the main drive. I will not use the machine for anything else as I conduct this test, so we can be (relatively) certain that it's the step of applying the OS update that is the culprit (if the symptom occurs again!).
I'll report back shortly, I hope.