You will live to be disappointed - the BIOS is nowt to do with selecting the OS.
Certainly you seem to be right with respect to modern BIOSes. I see no way at this time to achieve my desired results. However, should I be able to cause the computer BIOS to read from a
selected physical drive, then it could only load the OS on that drive.
The problem is, that Dual Boot techniques, leave a certain intertwining of the drives that's there until shut-down. Everything I glean from your link substantiates this.
I want the drives to have total and utter autonomy.
Way back in early W98 days, I used to call on whichever drive I wanted during start up. It had the unanticipated and extraordinary characteristic of allowing one to see into the dead drive. So, it wasn't as dead as the setup seemed to imply. Furthermore, one could write files to that second drive.
When the next Windows offering came along, this was totally changed. The second drive could be used as a backup, but until it was called into play on boot up, it was invisible to the current system. This was a great disappointment, as the first set up had huge customer appeal for CAD workstations.
I could resort to swappable carriages, but would prefer not to, I'm even considering fooling the BIOS into thinking one drive or another is not there on start up. This means cutting the power or a critical line to the unwanted drive, but what a palaver. It's a pity that A Meggatrends and others can't give a choice on start up.