Asus P8Z77-V Pro: UEFI boot causes grey screen, F8 Boot Manager works fine

[truprecht]

MB: Asus P8Z77-V Pro
GPU: MSI HD7950 Twin Frozr OC
OS: Win 10 64
Boot SSD: Crucial M4 128GB

After I upgraded from Win 7 Pro to Win 10, I would get a grey screen after the blue window logo with the 4 rotating balls. I assumed it was a GPU driver issue, and spent weeks messing with drivers, driver cleaners, safe mode boots, CCC install failures, clean Windows re-installs, etc. All that effort has come to this:

If I allow the computer to boot "naturally" to the UEFI P1 Windows Boot partition, I get the same grey screen. However, if I go to the BIOS and use the F8 Boot Manager, the computer works perfectly. I have the most recent BIOS on both the MB and GPU.

How can I get it to boot correctly without having to use the BIOS boot manager? Is this related to the UEFI Secure Boot "feature"? Is there any risk to disabling Secure Boot? Did I select the Windows install partition incorrectly?

 

[smorizio]

make sure the mb has the newest windows bios in cap form on the mb to rule out a bios bug. on the z77 there the first mb to also have muilt monitor support in the bios. under primany display set it to peg/pci if your using a gpu and under it turn off muilt monitor support. if it still errors out. try setting the video and other pci slots to pci 2.0 mode see if the mb has a bad plex chip.

 

[truprecht]

make sure the mb has the newest windows bios in cap form on the mb to rule out a bios bug.

What do you mean by "bios in cap form"? I have the latest version Asus offers for the MB (version 2104) but it's from 2013.

http://www.asus.com/us/Motherboards/P8Z77V_PRO/HelpDesk_Download/

I'll look at the other settings - thanks for the leads.

 

[truprecht]

Update:
Turned off multi-GPU (Lucid Virtu MVP) in the BIOS. No Change.
Changed the primary GPU selection from "Auto" to PCIE. No Change.
Disabled Secure Boot by clearing the keys. No Change.
Changed the boot OS type from "Windows UEFI" to "Other OS". No Change

Still works fine if I manually select the boot device, still goes grey if I let it automatically boot to the exact same device.

 

[truprecht]

SOLVED. In the BIOS I turned OFF the Compatability Support Module (CSM) and then it started booting correctly. When the CSM was on, none of the 3 settings (UEFI only, Legacy only, UEFI & Legacy) allowed it to boot properly.