[SOLVED] The Energizer Bunny is haunting my computer

[mcbenney]

The MB is an old Asus Sabertooth X79 purchased in 2011.
System PSU is an equally old Corsair HX850.
GPU is an Evga GTX 480
Case is the only new hardware component, a Fractal Design Define C

OS is Windows 10 Pro, clean install, up-to-date
BIOS are up to date (v.4701)
BIOS have been reset to default settings
Device Manager shows no errors
System Panel power connectors are connected correctly.

There are two case fans, the CPU fan, GPU fan and PSU fan. On the mother board itself there is also a I/O cover that has a fan and there is a fan over the PCH.

So the weird thing that is happening is that one of the case fans (the front), the I/O fan, and the GPU fan continue to run after the system is shutdown. (This is a real shutdown. CMD: shutdown /s. Not just pressing the power button.) The CPU fan doesn't continue to run but it will jerk every two seconds like it's trying to start spinning but can't quite overcome its own inertia.
If I leave the computer turned off but plugged in, the fans continue to run…forever.

The fans also continue to run if I unplug the PSU and flip its power switch to 'off.'
They continue to run if, after unplugging the PSU, I press the case power button in an attempt to drain the system capacitors.
Under these conditions they will keep going and going for 5... 6...7 minutes.
Eventually, they will stop because, well… the first and second laws of thermodynamics can't be denied.

I've plugged both the CPU fan and the front case fan into different fan headers on the board. No change in their behavior.
So what gives? The Google says that this is an out-of-date BIOS issue for this MB but I’ve addressed that. Any other suggestions?
The motherboard has a standby LED that remains lit after shutdown, indicating the the motherboard at least doesn't think the system is fully shut down. How can I convince it otherwise?

Thanks in advance for any insights. Have a safe and happy New Year!

 

[popatim]

Now there's a weird problem...
Have you tried holding down the shift key while click the Shutdown button on the start menu?
I'm thinking there's something wrong with your PSU.

 

[mcbenney]

Now there's a weird problem...
Have you tried holding down the shift key while click the Shutdown button on the start menu?
I'm thinking there's something wrong with your PSU.

Thanks. Tried that. No change in behavior.

 

[mcbenney]

Problem fixed but mystery not necessarily solved.
Tried to isolate the problem to the motherboard by unplugging all peripherals. First, I unplugged drives then booted. Naturally, system couldn't find OS drive so I pressed and held the power button to turn it off. Fans, including the GPU fan, keep running. I decide to go ahead and remove the GPU PCI-E power connectors figuring that will kill the GPU fan making it safe to remove. Interestingly, it does not. Even without PCI-E connected, GPU fan keeps spinning. I decide to give it the requisite 6 or 7 minutes to drain power and then remove the GPU. In preparing for that step, I unplug the DVI video cable from the GPU. The instant I do that, ALL THREE fans stop spinning.
That's odd. Coincidental timing perhaps?
I plug the DVI connector back in.
Fans start up again.
Pull it out.
Fans stop.
Very odd.
Anyway, to make a long story short, I ended up physically removing, power cycling and then re-installed the original GPU but for some reason the system drivers no longer recognized the card and I had to completely uninstall and reinstall the NVIDIA drivers (using the same download I had used originally, incidentally--version 391.35).
The only difference is that this time I opted not to install the 3D Vision Drivers.
When all that was done, I did two or three more test shut downs. In every instance, all fans stopped spinning when shutdown was complete.
So problem fixed but mystery remains. Definitely an issue with the GPU, but what? Perhaps the card wasn’t seated properly? (I don’t think so.) Maybe the card’s contacts were dirty? Perhaps the first driver installation got borked somehow? Or maybe the 3D vision drivers that I didn't install the second time 'round were the culprit? I'll never know for sure.
In any event, “mischief managed.”