Update: SOLVED. See bottom post.
I'm using an Asus ROG Crosshair VIII Hero (wifi) motherboard v.3801.
Right now, I cannot boot from a live bootable USB flash drive.
Two days ago, I could.
Since then, some BIOS / UEFI settings have obviously been changed, but I can't seem to get them all set right again so that I can boot from a flash drive.
[Update: SOLVED, see below.]
The board does recognize the flash drive, and the drive's icon appears on the desktop after booting into my OS, but I can't boot into the flash drive.
It's plugged directly into one of the motherboard's blue 2.0 USB ports.
My OS is Ubuntu-MATE Linux 20.04.3.
My question is: What are the key necessary BIOS / UEFI settings I need to boot from a USB drive? CSM is enabled, all the individual USB ports are enabled. SecureBoot is off.
Thanks for any tips.