Process goes like this:
- boot Windows installer USB
- at first screen press Shift+F10 to get command prompt (hit enter after each line to type)
- type: diskpart
- type: list disk (so you're certain of the disk you're working with, say it is disk 0)
- type: select disk 0
- type: clean (don't blame me if you don't know what this does
- hope there was nothing on that drive you wanted to save)
- type: convert gpt
- type: create partition efi size=200
- type: format quick fs=fat32 label="EFI"
- type: exit
- type: exit