Resolved in my Power Mac 7200 with a green Sil3112 SATA card: Per the second paragraph in
this site evidently Mac OS 9 can't boot from a volume size above 190 GB *. I have a 256 GB SATA storage device in my 7200, so I reformatted it with a Mac MDD running OS 10.5.8 in HFS+, no journaling, with Mac OS 9 drivers, this time into three user partitions for an initial experiment. Now my 7200 boots from the device perfectly smoothly (and clearly faster than from Blue SCSI).
Previously, with just one user partition, my 256 GB SATA storage device operated normally and reliably except that it wouldn't boot with OS 9.1. So I'll probably revise my partition scheme into two user partitions, one of just 500 or 750 MB for the boot volume, the other with all the remaining capacity.
For my PowerMac 9500/120, from a ChatGPT response I edited:
9500/120 PCI bus slot groups:
A1, B1, C1 (32 bit PCI slots): 32 bit PCI slots typically used for various expansion cards such as graphics, network, or other cards.
D2, E2, F2 (64 bit PCI slots): 64 bit PCI slots which provide higher bandwidth. Some specialized cards may be designed for these slots.
I only tested one 64 bit slot but I suspect all are dysfunctional with all external devices connected to my red SATA PCI card. But the 32 bit slots seem fully functional, including bootable, at least for volumes of 190 GB or less (I've not tested booting my 9500 with larger SATA volumes yet).
The inexpensive green Sil3112 SATA cards from China provide no eSATA connection nor provisions to add one, a shame. I revised one by creating a rectangular hole and two bolt holes for a panel mounted eSATA to SATA cable, then installed the cable, completing an external eSATA receptacle. I ordered 7 more such cables
here. It should be easy to clamp a side guide to a drill press table to render milling semi-circular ending slots in several panels quickly and easily, and of course drilling the two bolt holes is quick and easy. (With the circuit board removed for the work of course, then replaced.)
I'm preparing several because eSATA seems like the best option for moving data between classic systems with a bus which can host a SATA card. It's fastest, seems most solidly reliable, covers all systems with accessible buses except NuBus, and is easy to use with modern systems as well (via USB with an ordinary bridge case or cable if the system provides no eSATA ports). For SCSI systems with NuBus or no accessible bus, BlueSCSI or SCSI2SD seems like the only practical option, but for all else eSATA seems clearly best to me. Many kudos to DosDude1 / Collin and all other noble contributors for making this superb option practical!
I still need to upgrade both systems from Mac OS 9.1 to 9.2.2 and render some other refinements. And find a way to install Collin's cool firmware into my Sil3114CTU SATA cards then test those. I'll report again as new results arise. Cheers!
* I don't recall whether I've tried booting in OS 9 from a Blue SCSI volume larger than 190 GB.