Well, I went into HDToolBox, changed the boot prioritiy from 3 to 1, saved, and exited. Then I rebooted with the Jaz drive connected and turned on (and the light at the back was lit, indicating termination).
Nope, I still got the Commodore "insert WB disk" animation.
Early Startup did not show the boot drive (HD0:), although it did show all three partitions of the second drive (all non-bootable).
SysInfo soon froze while trying to find the SCSI ID of the 2nd drive. I wonder if perhaps it isn't connected to the IDE drive, since I vaguely recall now that the original 120MB IDE drive was removed when the 2nd drive was installed, so it wouldn't be a SCSI drive after all.