I don't know how it happened, but it's not letting me convert my SSD to GPT because there's a system partition on a HDD that I only use for games and stuff. I added in this drive a couple months after building the PC, so there's no way there should be any system files on it. (Also the numbers got swapped for some reason, my main SSD is now Disk 1 and my backup HDD is Disk 0). Unplugging the HDD leaves my PC unable to boot, even though I have my SSD set as the boot drive in BIOS.