Hi,
I'm just now trying to do a similar upgrade, but I'm stuck kind of early on...
I stopped keeping this system up to date, so I'm re-learning a bunch of stuff...
I updated the BIOS to UEFI U1N
I created my install drive with UniBeast using High Sierra/UEFI Boot Mode.
My problem is that I can seem to boot the USB drive fine, but it never lists 'Install High Sierra'.
I can boot my old El Capitan drive and that seems to work ok, but the High Sierra one just shows a clover logo and no drives to choose from.
I'm quite likely missing something stupid, but I'm kind of stuck, so I figured I'd ask for help in case this is something common that I've managed to miss.
This is the basic specs:
Motherboard: Gigabyte GA-Z68X-UD3H-B3 BIOS U1N
CPU: Intel Core i5-2500K
Video: Gigabyte Radeon HD 6850 1GB
When I stop the boot to choose a boot device, there's a long list, but PMAP appears to boot my drive. I don't see an entry for USB:NameOfDrive, which is partly making me think I've got a BIOS setup issue. I did try to 'load optimized defaults' but that didn't seem to make a difference.