GRUB Doesn’t detect windows: Disk is MBR but booting via UEFI

I dun goofed. I thought my Windows was booting using UEFI, and I installed Ubuntu using UEFI. But now I think my Windows was booting using MBR, and GRUB can’t detect it.

I’m confident the disk is MBR:

> sudo gdisk -l /dev/sdb GPT fdisk (gdisk) version 1.0.3  Partition table scan:   MBR: MBR only   BSD: not present   APM: not present   GPT: not present 

and that I’m booting via UEFI:

> ls /sys/firmware/efi/ config_table  fw_platform_size  runtime      systab efivars       fw_vendor         runtime-map  vars 

How can I fix my GRUB so that it boots using MBR instead of UEFI?

I’ve tried:

  • Running os-prober and update-grub
  • Running boot-repair

Neither gave me a GRUB menu with Windows listed.