Great trouble shooting thread. Excellent advice here. Hope it all works now! 😀

I got a Highpoint UDMA controller running in my retro machine (pentium).

As for my experience with it:
You have to have a BIOS that supports boot from SCSI to boot from a HDD on the controller.
SCSI means infact something like 'external controller', it doesn't matter which type.
The exact HDD to boot from is set in the BIOS of the IDE Controller.
There might be the possibility that if you disable the internal IDE controller the BIOS tries to boot automatically from the external controller. I haven't checked this.

The controller does not support ATAPI, so the DVD-drive is attached to the onboard controller.