I am running a dual boot Windows 10/Ubuntu 16.X on an ASUS Zenbook UX330U. After a Windows and possibly BIOS update yesterday, the option to dual boot is missing.

BIOS Home Screen

I can only select Windows Boot Manager. I recall having this problem before, and it was easily fixed by repairing GRUB using a LiveUSB. However, I cannot get the LiveUSB to boot. I disabled Fast Startup in Windows 10. I have disabled Fast Boot in the BIOS. Legacy USB has been enabled. The CSM option is greyed out, and I cannot turn it on.

Further info from comment:

I didn't do anything. Both the Windows and the BIOS update seemed to occur automatically. The only reason I suspect a BIOS update is that my ASUS booted to the BIOS screen after the first update mandated reboot. Then it proceeded to Windows Boot manager, updated, and rebooted again. The next reboot went right to Windows, updated, the rebooted. This process repeated three or four times. When I couldn't boot to Linux, I checked the BIOS version number was 315. I don't recall what it was before

Fast Boot Disabled

Legacy USB Support Enabled

Any help getting a LiveUSB to boot so that I can repair my GRUB so that I can go back to using Ubuntu is greatly, greatly appreciated.