You get a new motherboard with an Intel 500 Series chipset. You decide to just use the integrated graphics in the CPU, but you can’t boot!
It turns out your actual boot drive is missing from the Boot options menu. Gonzerooni!
I recently experienced this problem on a Z590-based motherboard.
This is what I saw (top) and what I should have seen (bottom):
No matter what I did, I couldn’t get the new motherboard to see the mirrored linux boot drives…
It’s pretty simple. The Asus support page includes the following note:
1. Q: When I use the integrated graphics card on the Intel® 500 series motherboard , why does the CSM option under BIOS appear gray and non-configurable?
A: The Intel® 500 series chipset does not support UEFI VBIOS graphic card, hence the integrated graphics mode does not support legacy boot and CSM option becomes non-configurable
2. Q: How can I configure CSM normally?
A: You can use a discrete graphics card that supports UEFI VBIOS to make CSM option configurable
In other words, Intel decided not to support integrated graphics that boot the old MBR/BIOS way with their newer chipsets.
Your only option is to use the GPT/UEFI way of booting – if you want to use the integrated graphics.
If you don’t know the difference, check out my explanation of BIOS vs. UEFI and MBR vs. GPT here: Convert your Windows 10 boot drive from MBR to GPT
So, either you convert from MBR to GPT (see link above), or you have to install a dedicated graphics card. Note that an older VBIOS graphics card will work just fine!
After you do one or the other, POOF…
CSM will be enable-able again, and your MBR boot drive will once again appear in the Boot menu!