CMD prompt (in admin mode)
Incidently don't confuse SECURE BOOT with UEFI. You CAN of course boot a UEFI enabled W10 -- SECURE BOOT has to be disabled for booting OS'S not stored in the BIOS for example if you want to boot from a USB or boot say a Linux system.
Even on NON UEFI systems I'd make all HDD's GPT apart from the boot one. On a non UEFI system you can't boot from a GPT HDD but all other HDD's can be GPT.