My MSI Z390 board is set to boot from Legacy + UEFI, but when I converted my MBR drive to GPT it didn't recognise it as a UEFI drive, despite it showing up as GPT in DiskPart. The same thing happens when its set to boot from UEFI only.
Is this most likely just a procedural error in the conversion step or something else? I reinstalled w10 from scratch after not being able to boot from the drive, and weirdly it didn't let me install on that disk because it was GPT – and my bios was still set to Legacy + UEFI.
(I converted using the aomei partition assistant because for whatever reason the mbr2gpt tool didn't want to work)