can't boot from underlying volume but can from Sandbox

[camner]

Wow, that was a quick reply! Thank you.

Sorry, but I'm a bit confused. The original drive is the one that I can't boot from (but I CAN boot from the Sandbox made from the original drive).

Are you suggesting that I reclone the original (non booting) drive to a new volume somewhere, and then clone back? What am I misunderstanding?

FWIW, I have a clone I made from the original volume shortly after creating the sandbox. That won't boot either (same issue...machine shuts down).

Right now, the only volumes I can boot from are the Sandbox and a month old backup (clone) of the original drive.

What would be the effect, if any, of "copying back" from the Sandbox to the original drive that now won't boot? Is that worth trying? Or are things likely to be sufficiently FUBAR that I should go back to the month old working volume and just rebuild forward from that?