Odd behavior when one specific drive is connected

I came across a weird problem with a customer computer…

Quad core AMD on an Asus MoBo.

Running Win7 Ultimate (actual legit copy, with a real COA sticker even)

the computer would not recognize the optical drive or for that matter ANY optical drive… So I’m thinking high and low filter issue in registry after nothing else works…

So I do the regedit exactly to the M$ instructions, check everything three times before I save it… and reboot the computer… won’t start.

It dumps me to startup repair, after five and a half hours chewing it finally tells me it can’t repair… “Do you want to try system restore?”
What the hell… try that and “No OS found”. curiousier and curiouser…

At this point I feed it a Win7 Ultimate install disc and choose the repair
option… still won’t start. Now I’m getting bugged.

My next reaction is part of my normal modus operandi… I disconnect
the customers drives (all of them) grab a spare out of my collection
connect it to the #1 channel and feed the computer the Win7Ultimate disc again, but this time I let it install the OS on my bare drive.

I make “disposable” OS installs all the time

Oddly all the hardware works perfectly… Ok that’s
one issue I don’t have to deal with.

Verifying hardware is WHY I make “disposable” installs

Let it run updates, install the basic suite of stuff I
install on EVERY computer it all still works…

Now the “weird” part
BUT when I shut down, connect his drive (to channel #4)
and try to restart with his drive as a data drive it again tells
me “No OS found” disconnect it and restart my temp install
on my drive starts fine…

Someone out there is thinking “Signature collision”.

Nope. a signature collision would let the computer start,
but the drive would be “invisible” in windows, except in drive management where it would be flagged as “offline”

Install his drive into an eSATA enclosure and connect it
to my desktop it’s all perfectly accessable. I can access
and write anything to and from the drive.

I’m long past the point of trying to fix this broken OS
both the customer and myself will be completely
happy recovering his Data & Documents

And at this point I’m curious what blocks the computer
from starting with this drive connected, when it is NOT
the active system drive.

I can understand it not starting if it was connected to the lowest
numbered SATA channel, IF there was not another functional OS
drive connected to channel #1.

I keep thinking there’s something that my brain is not processing after three days of eating turkey…