OK, after more rigorous testing I’ve found that the PCI SATA card just isn’t terribly reliable in this specific machine. I may test it in my G4 MDD at some point to confirm if it’s the card or just the combo of the card and a Power Macintosh G3 B&W. Some things I’ve tried that ultimately didn’t fix the unreliability of PCI SATA boot: having a drive connected with IDE, restarts versus cold starts, the CUDA switch, resetting PRAM three times, open firmware reset-nvram set-defaults, removing the unused Grappler SCSI card that was in this G3 when I got it, moving the SATA card from PCI slot 3 to 2, and booting with different startup volumes selected.
There are two states the computer can boot into. In one, the PCI SATA card is recognized at boot, the startup disk (whether IDE or PCI) is booted from successfully, and all IDE and PCI volumes are mounted on the desktop after boot. The second state occurs seemingly at random. In the second state, if the startup disk is IDE, the computer boots normally but the PCI volumes are not mountable at all. SCSIProbe does show the PCI SATA drive, but no volumes are readable. If the startup disk was a PCI volume, the floppy ? icon appears. If there is a bootable IDE drive connected, the first bootable volume on that drive is automatically booted after a few seconds of the flashing floppy icon. If no IDE drive is connected, the flashing floppy icon lasts forever.
So essentially, the PCI card randomly doesn’t work on some boot attempts. There is no apparent trigger for this behavior. The only thing that consistently works is shutting down and trying again. It may take one or a dozen attempts, and then it may continue working just fine for one or a dozen boots, but nothing is permanent. I will be using the startech ide to sata adapter because it has worked every boot.