Change from UEFI to Legacy

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Solution 1

I advise against doing what you're asking to do, particularly in light of your comment in response to the question of why you want to make this change:

Just don't care about it.

If you don't care about it, then the saying "if it ain't broke, don't fix it" applies. Absent a compelling reason to do so, switching from EFI-mode to BIOS-mode booting offers no benefits, but poses several drawbacks, including:

  • Risk -- Such a conversion runs the risk of creating new problems you're not experiencing now. These risks include damage to installed files and the possibility that you'll end up with an unbootable (or even completely trashed) system.
  • Time -- It will take time to perform such a conversion. You'd probably be better off doing something else.
  • Slower boot -- Although this isn't universally true, EFI-mode booting is usually slightly faster than BIOS-mode booting.
  • Reduced functionality -- You might not care about it, but booting in EFI mode provides you with some minor features that BIOS-mode booting does not provide. Most importantly, you can install multiple boot loaders and change which one is active with a single command. EFI boot variables and access to EFI system calls provide the potential to do more, although these features have yet to provide significant benefits in Ubuntu.
  • Lack of Secure Boot -- This much-maligned feature actually provides some modest benefits because it can prevent pre-boot malware from running. To be sure, not all EFI-based computers support Secure Boot, but just about any desktop or laptop computer that shipped with Windows 8 or later does.
  • Complications to current and future multi-boot configurations -- If you're currently multi-booting with other OS(es), switching from EFI-mode to BIOS-mode booting requires converting all your OSes. (Well, there are ways to boot different OSes in different modes, but they're tedious.) If you're currently booting only Ubuntu, enabling the Compatibility Support Module (CSM) in the firmware, which is required to boot in BIOS mode, makes it more likely that you'll run into problems when installing new OSes. See my page on this topic for all the gory details.

To be sure, there are scenarios in which switching to BIOS-mode booting makes sense. You, however, have provided no such compelling reason; you "just don't care about it," to repeat your words. Given the preceding list of drawbacks to such a conversion, "just don't care about it" does not qualify as a compelling reason to take the risks and accept the drawbacks of switching to BIOS-mode booting.

If you insist on ignoring my advice, you can do the conversion. In brief, you must enable your computer's CSM and install a BIOS-mode boot loader for Linux. The most common of these is the BIOS-mode version of GRUB. The Boot Repair tool will do this in a semi-automated way; however, you must boot the OS used to run Boot Repair in BIOS/CSM/legacy mode, and this in turn requires either luck or that you understand how to control the boot mode. See my page on the CSM, referenced earlier, for information on this topic. If you use GRUB, you should also create a BIOS Boot Partition on your disk (it probably does not exist now).

Solution 2

I recently has a need to switch from UEFI to legacy boot, and I found this thread. Needless to say a non-answer which I found here irked me, and I wanted to document an actual conversion. Here's how I actually did it:

Install legacy boot support:

apt install -y grub-pc-bin

Comment out /boot/efi from /etc/fstab

it might give you a hard time when booting

Turn on protective MBR and configure grub partition bootable

Your mileage varies here, my disk had sda1 being EFI partition, and sda2 being /boot.

parted /dev/sda
   disk_set pmbr_boot on
   set 2 boot on
   q

Update MBR and regenerate grub config:

grub-install --target=i386-pc /dev/sda
update-grub
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Updated on September 18, 2022

Comments

  • FinKsu
    FinKsu over 1 year

    I have Ubuntu installed on my computer in UEFI Mode. I'd like to change to Legacy BIOS Mode. I need to know can I boot to UEFI Ubuntu on Legacy Enabled?

    • mook765
      mook765 almost 7 years
      Why do you want to change to legacy-mode? UEFI-mode has it's benefits. If the system runs nicely and you don't know much about this topic, I'd advice to leave everything as it is.
    • FinKsu
      FinKsu almost 7 years
      Just don't care about it. I want to know can I boot to UEFI Ubuntu with Legacy Enabled
    • pdc
      pdc almost 7 years
      if you follow the official Ubuntu advice on how to do this, it might give you best chance; many would echo mook's advice to leave as is; here is the link from ubuntu help.ubuntu.com/community/…
    • mook765
      mook765 almost 7 years
      In BIOS you should have three options for the boot-mode 1: UEFI , 2:UEFI+Legacy, 3: Legacy only. Ubuntu installed in UEFI-mode won't boot with option #3, but will boot with option #1 and #2.
    • FinKsu
      FinKsu almost 7 years
      I have HP 15 Notebook PC. It has Legacy Support, so is it #2 or #3?
    • mook765
      mook765 almost 7 years
      With option #2 you can but to Ubuntu in UEFI-mode or boot to anything else which is only able to run in legacy-mode (for example you want to boot from USB-stick, and this stick can boot in legacy-mode only).
    • mook765
      mook765 almost 7 years
      And why I shouldn't care, if I give you an advice which leads you to render your system unbootable it really wouldn't be nice! So it is better to ask for more information.
  • TamusJRoyce
    TamusJRoyce about 5 years
    Your suggestion to not do it should be summarized to: "clone your hard drive to a different drive before doing so. otherwise, you may corrupt files or cause other problems." His opinion of "I don't care" and your opinion of "don't do it" are border-line offensive. However, I have a great install of ubuntu. I have a legacy pc without uefi bios I have cloned to a different hard drive. And the last part was really helpful information. Thank you.
  • Markus Bawidamann
    Markus Bawidamann almost 3 years
    This is not answering the question nor providing a solution, it finds Arguments against the change, completely ignoring that this will be required from time to time and a legitimate thing to do.
  • Admin
    Admin almost 2 years
    Thanks alot @galets, this worked like a charme on Debian 11. Simple reason for switching from UEFI to legacy was that I had to build in the SSD into a PC without UEFI support.