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I installed Ubuntu in EFI by accident cause I taught erronously that Windows was also in UEFI and then lost access to Windows in the boot menu. I tried many tools like boot-repair (which did identify the problem immediately) which is amazing! However I was unable to fix the problem with it.

How do I recover Windows and get Ubuntu running 'alongside' in dual boot mode?

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  • UEFI has ESP - efi system partition which now will have boot flag. You need to move boot flag back to Windows primary NTFS partition with boot files, often sda1. Make sure Windows boots, turn off fast start up & make Windows repair flash drive. Boot Ubuntu live installer in BIOS mode, & Remove ESP partition with gparted and use Boot-Repair to install grub to MBR. Keep both Windows repair & Ubuntu live installer as Windows will turn fast start up back on and grub then will not boot Windows. Better to reinstall Windows in UEFI mode, as then you can always boot from UEFI boot menu.
    – oldfred
    Mar 20, 2021 at 14:02
  • I think mkusb has an option to install for BIOS only boot. If you boot the installer in BIOS it will install as a BIOS booter.. Mar 21, 2021 at 11:37

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Use at your own risk: My solution was to:

  1. Boot with the Ubuntu installation media.
  2. Go into "try Ubuntu" mode from that media.
  3. Download os-uinstaller which is an amazing tool!
  4. Use it to remove the new fresh Ubuntu installation and repair MBR.
  5. Boot back into Windows normally.
  6. Free up some hard disk space with AOMEI partition assistant (but probably any such tool).
  7. Hold down shift and hit restart and go into command-prompt from there to do: mbr2gpt /convert.

After that worked out luckily I could and did install Ubuntu again and it works OK.

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  • Thank you for letting us know what worked for you. Mar 29, 2021 at 7:20

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