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Old July 5th 18, 05:04 AM posted to alt.comp.periphs.mainboard.asus
Paul[_28_]
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Posts: 1,467
Default SOLVED: Problem with Asus Prime X299-A UEFI BIOS

Bill Anderson wrote:
I'll post this even though I'm not convinced anybody who can use the
info will ever find it here. It's just that I've received so much good
info in this newsgroup over the years (primarily from Paul) that I want
to give back if I can.

My problem was that I'd installed three versions of Win10 on my new
Prime X299-A setup and when I'd choose something other than my default
OS using F8, all the boot options (OSs and optical drive) would get
scrambled in UEFI BIOS. Once I'd picked an alternative OS using F8, I
never could be sure exactly what OS I'd get upon reboot. (I don't use
Boot Manager, want never to see it.)

I explained this problem today to a tech via the Asus website chat
feature and this is how he replied:

This should do it:

1. Disconnect all the other drives from the unit leaving only the main
boot drive.
2. Go to BIOS and under boot then go to OS type and set it as Windows OS.
3. Disable Launch CSM.
4. Save and Exit the and this should lock it as the boot device.

After this you can add back the other drives and try the F8 option and
see if it fixes the issue.

Yes, I had to do a little investigative digging to find a way to set OS
type as Windows OS, cuz the option wasn't exactly where he said it would
be, but that's OK. Bottom line is that this worked: Now I can pick the
OS I want by using F8 without worry.

And the takeaway is that Asus Tech Support came through for me on the
first try. Yay!


I find

powercfg /h off

works for me. This prevents both hibernation and Fast Start,
so that the BIOS F8 selection process will be an honest one.
I can leave CSM enabled.

Of course, if you're a big hibernation person, that'll be
an issue for you.

EasyBCD could be used to add the other OS drives, then leave
the BIOS configured to boot the "boss" drive. But that doesn't
cover every usage scenario, so won't keep everyone happy.

UEFI has a relatively large NVRAM storage facility. Likely
using the BIOS Flash chip for the actual storage. It can
memorize environment variables. By comparison, traditional
BIOS only have the 256 byte CMOS storage. And using CSM mode
is an emulation of a traditional BIOS.

Intel has declared CSM to be a dying feature, so
in a few years, motherboards will be UEFI only.
We'll have to wait and see, as to whether the rest
of the industry agrees.

Paul