As a followup, I tried to run a combo updater install to correct this issue. I booted from my External HD, and downloaded 10.6.8 to update the non-bootable drive. However, 10.6.8 says it cannot be installed on this drive:
I read that 10.6.5 changed something about the partition table, so I tried falling back to 10.6.4 instead, and got the following message at the end of the install process:
I then tried 10.6.5 which says the disk is not properly configured to accepted the installation:
So the non-bootable drive has 10.6.3 installed on it, and 10.6.8 cannot be installed because it is not the startup disk, 10.6.5 cannot be installed because "the requirements are not meet" and 10.6.4 fails during the install process. Talk about nothing going my way!