The I would reformat the LaCie and see if that solves anything.
You might need to do an archive and install. The weird thing is... by the time it reaches the Apple, the system has already chosen a start up volume. But you say you can get it to start by going selecting the volume in multi-boot mode. So that is odd.
It's most likely a corruption in the system file that selects start disks, etc... but troubleshooting that would not be worth the time. I would do an archive and install.