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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.

Joey Eremondi
  • 679
  • Windows in MBR only boots Legacy/BIOS. You have to, maybe, install Grub to the MBR? I'm not sure if that's enough to "convert". Either way, it's preferable to install in proper UEFI mode any OS but especially Windows 8 or newer if the machine is UEFI. –  Jun 19 '19 at 14:52

1 Answers1

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So, it seems like GRUB was properly installed on the MBR, but the BIOS was not booting in the right order. These were the steps to resolve:

  1. Change boot order in BIOS to boot from MBR (for me, this was choosing the option for the Hard Disk that didn't say UEFI).
  2. Boot into Ubuntu.
  3. Run sudo os-prober
  4. Run sudo update-grub
  5. Reboot
Joey Eremondi
  • 679