From happy Mac to flashing question mark during boot?

[obsolete]

Following this post, I decided to start a separate thread about this specific phenomenon to try to understand what's going on.

Here's what's happening:
The machine boots to a flashing question mark, then quickly switches to a happy Mac, then back to a flashing question mark, where it sits forever. The fact that it shows a happy Mac icon tells me that everything has gone right up to the point of finding a blessed system folder to boot from, but then something goes wrong that results in the machine returning to displaying a flashing question mark. I'd like to understand what is going wrong here, but I'm not sure how to begin troubleshooting.

Here's the setup:
- Power Mac 5400/200, trying to boot Mac OS 8.6 from a 4GB partition on a SATA drive attached to a System TALKS PCI card with a VIA VT6421A controller chip
- When attached to a PCI card with a SIL3112 controller chip (flashed Adaptec 1210SA), the 5400 will reliably boot 8.6 from the same SATA drive
- When the system is booted from another drive (on the onboard IDE interface), the drive on the VT6421A card shows up and works well in OS 8.6; it can be initialized, read and write, etc.

 

[adespoton]

Does the drive have a System folder that is not only blessed, but functional? That behaviour is something I've seen regularly where the system folder itself was corrupted. Blessed, so you get the happy mac, but then it fails the bootstrap process, so you don't get the "Welcome to Macintosh" screen and it goes back to a ?.

This is different than when it switches to a happy mac, then goes blank, then has a disk with an X, then goes back to the flashing ?.