[wtallis]

> But I think to boot from an unexpected AHCI controller you might need a boot rom?

The whole point of AHCI was that you wouldn't need a boot ROM. You already have firmware support for talking to an AHCI controller to find drives and locate an operating system, because that's how the system boots off the built-in SATA ports. The firmware modules required to do the same with an AHCI add-in card are the firmware modules you're already using, so the add-in card doesn't need to bring them along in an option ROM. Same for drivers in the OS.

Most likely, the motherboards in question simply never implemented code to probe for any other AHCI devices beyond the built-in one, and would have been equally unable to boot from an AHCI controller card providing more SATA ports.