Startup Disk disagrees with restart options, and restart options is right

I've had a heck of a time getting the Ts crossed and the eyes poked 
(ow!) installing High Sierra on an internal OWD SSD and all the external 
backup drives such that they all boot. i'm almost done. It's taken 
several days. APFS has been a chore.

I have a 4TB external hard drive that has High Sierra installed, is an 
HFS+/GUID drive. It shows up in Preferences>Startup Disk as bootable. It 
does not show up in the restart options menu, and the MBP doesn't boot 
from that drive when selected via Startup Disk.

Anyone know what's going on with this? Regrooving a 4TB drive is tedious.

Mike