Running bless to place boot files failed

Customer want's drive partitioned to be able to boot from 10.13.2 and 10.6.8.

I installed a 1TB Seagate FireCuda with the OWC thermal sensor. Prior to installation I had the new drive in an external enclosure hook up to a Mac Pro 2nd generation. Formatted the drive and gave it two partitions. Installed 10.13 and then booted from it and ran updates to 10.13.2. This machine shipped with 10.6.6. So I could not boot from a retail Snow Leopard 10.6.3 and update. So I pulled a drive out of an older MBP with 10.6.8 and used SuperDuper to clone the system onto the the second partition. Customer needs 10.6.8 to run old Adobe programs. He also has a Time Machine backup of the old drives. Both were loaded with viruses which I cleaned off before using Migration Assistant.

The problem is I can't select the 10.6.8 partition to boot from the internal disk I cloned from externally and get the message " You can't change the startup disk to the selected disk." Then message "Running bless to place boot files failed. I have run both Disk Utilities and Tech Tool Pro volume rebuild. It will boot from the 10.6.8 external disk I cloned from. If I select the internal 10.6 drive from start up preference, it will start to boot give circle with a line through it and then go to the 10.13.2 and boot. Don't know where to go now.

@dan @oldturkey03