^^^ Gotta wonder how this relates to the question ask... "How do I know if I should do UEFI mode or Legacy mode when making the USB?" If your board supports UEFI, build the installer UEFI.
No, one looks at all the options and makes an informed decision. Learning the UEFI Shell is not easy. Install Windows onto another drive or onto the same drive and the install, or an update (say from Win8 to W10) could bork the UEFI boot loader; install Linux and it could bork the Windows and Clover boot loader, play around with Clover Settings and/or play around with Clover themes and you could bork Clover boot loader. Play around with Linux and you may blow up GRUB2. Re-installing it could be easy or it could be a nightmare, especially if you use the Ubuntu GRUB repair tool. You're running fine and Windows burps and you're now in a world of hurt... BTDT.
I use Legacy. Windows has its own Legacy Boot loader. Linux Mint has it's own bootloader, which allows me to boot either Windows or Linux. OSX has its own boot loader. Chances are that before I allow Windows 10 Anniversary update to install I will disconnect my Linux and OSX drives so that it doesn't bork them. I couldn't do that if I used UEFI. UEFI writes out to the mobo NVRAM. It gets borked and you're going to learn UEFI Shell the hard way. Hope you have a second PC to be able to get help...
-----------------------------------------------------------------------------------------------------------------------------------------------
LBD,
I don't think it helps to have the GPU installed and not connected to power.
As far as the nv-disabled=1 flag goes you can do it one of two ways
1] When you get to the Clover boot splash screen it will default to the OSX HFS+ drive. Use the right arrow to get to the Options (iirc) (there might be two Options icons - you'll have to try both of them) icon, then scroll down to the boot flags section. This is where it can get hairy - many have problems inserting the text and getting out - it may jump all over the place, it may wrap around, it may over strike what you've input. I get the cursor to the end of the line then hit the "End" key on the keyboard, then use the arrow key to get in position, type in the text, then use the cursor to get back to the end (but not at the space after the last character) and hit the "End" key to exit, then use the up/down keys to get to the Return line and hit enter. You may need to also use the "Home" key to get to the beginning of the line.
2] Since you can boot into OSX you could download "Clover Configurator". You'd run EFI Mounter to mount the EFI partition then navigate to the Clover/EFI/Boot/config.plist file and double click it. You would then enable nv-disable=1 flag in the BOOT section, exit (and it should write the modified config.plist file out to the EFI partition.) You would then Shutdown the PC, re-install your GPU, connect the GPU power harness, power up the PC.
http://mackie100projects.altervista.org/download-clover-configurator/
See those three config.plist files?... It's better to just navigate to the correct .plist file and double click it.
