you have to boot off the cd (any bootable os install or update cd that is compatible with the machine) and then go under "Control panels" and select "Startup disk" and choose which partition to boot there
thats how it was done on powermacs that came before the invention of the Sawtooth AGP G4 which introduced the option key being held down to choose partition regardless of toolbox image (tbxi)
i think there were some number key shortcuts u could make which also would direct the g3 to boot off different SCSI iD's on your SCSI chain because at the time of the B+W g3 many "professional" mac's had been running off SCSI hard disks because they were much more reliable then ATA IDE hard disks of that age. i was wondering because the seritek 1s2 pci sil3112 controller comes up as scsi drive maybe it would be possible to use the scsi keyboard shortcuts built into macOS startup to be able to choose startup disk that way
"For classic Apple Macintosh computers, to boot from a specific SCSI ID, use the Command-Option-Shift-Delete-# key combination, replacing '#' with the desired SCSI ID number.
-To bypass the startup disk and boot from the first available bootable device or the internal CD-ROM, use Command-Option-Shift-Delete without the number. "
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