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I have a linux box (Ubuntu 16.04) whose boot disk is partitioned using MBR. How can I convert it to GPT+UEFI?

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Before starting, make sure you have a backup, and make sure to have a linux live boot ready to rescue your system. It's easy to mess this up!

  1. Use gdisk to convert the partition table to GPT.

    gdisk /dev/sda

  2. Create the "BIOS boot" partition that GRUB needs.

    n to create a new partition. Needs to be about 1MB. You can probably squeeze this in from sectors 34-2047. Use L or l to look up the code for "BIOS boot" (ef02).

  3. Write the new partition table.

    w

  4. Reload the partition table.

    partprobe /dev/sda

  5. Re-install the GRUB boot loader using the new partition scheme.

    grub-install /dev/sda

    Optionally reboot to verify it's working. If you just need GPT and not UEFI, you can stop here.

  6. Use gdisk to add an "EFI System" partition (ESP). Officially should be 100-500MB, but mine only used 130kB. Can be anywhere on the disk, so consider putting it at the end if you're using non-resizable media like a physical disk.

    gdisk /dev/sda and use n to create the partition.

  7. Give the ESP a distinctive label without whitespace like EFI-system, because we'll reference the partition label in fstab.

    c to set the label.

  8. Write the partition table.

    w

  9. Reload the partition table.

    partprobe /dev/sda

  10. Build the filesystem for the ESP.

    mkfs -t vfat -v /dev/disk/by-partlabel/EFI-system

  11. Create the ESP mount point.

    mkdir /boot/efi

  12. Add the ESP to /etc/fstab. It should look like this:

    /dev/disk/by-partlabel/EFI-system /boot/efi vfat defaults 0 2

  13. Mount the ESP.

    mount /boot/efi

  14. Install the GRUB EFI bootloader.

    grub-install --target=x86_64-efi /dev/sda

  15. Reboot.
  16. Change the BIOS from BIOS boot to UEFI boot.
  17. Use the one-time boot menu to force boot the disk. You may have to navigate to the disk -> EFI -> ubuntu -> grubx64.efi.
  18. Re-install GRUB's EFI bootloader to update the UEFI boot selector.

    grub-install

Resources:

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  • The BIOS boot partition is only needed for doing legacy boot on a GPT partitioned disk. If you boot with UEFI, it is not needed and will not be used. – Michael Hampton Apr 16 '19 at 1:40
  • You may also have to change your firmware settings to support UEFI booting. – fpmurphy Apr 26 '19 at 19:40
  • 2
    It should be noted that maybe you should install grub-efi-amd64 package before you get started with this, otherwise you can end up in all sorts of tinkering with chroot in order to be able to install grub and boot (like I ended up) – jsaddwater Jan 23 at 20:00
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I was converting my BIOS/MBR system HDD to UEFI/GPT without data loss

I booted in BIOS legacy mode with MBR boot disc.

As said, I shrunk last partition to create space for new EFI System partition with gParted and formatted it to FAT32.

Then with gdisk addded this new partition and converted MBR boot disc to GPT, without data loss (two root and one home partition):

Number  Start (sector)    End (sector)  Size       Code  Name
1            2048        40003583   19.1 GiB    8300  Linux filesystem
2       956772352       976771071   9.5 GiB     8200  Linux swap
3        40003584       893857099   407.1 GiB   8300  Linux filesystem
4       894054400       956465151   29.8 GiB    8300  Linux filesystem
5       956465152       956772351   150.0 MiB   EF00  EFI System

I mounted this Partition as (without any fstab modifications):

 sudo  mount /dev/sda5  /boot/efi

And then installed GRUB-EFI:

 sudo grub-install --target=x86_64-efi --efi-directory=/boot/efi /dev/sda

I Booted with Legacy BIOS and of course didn't have efivars, so got errors...

But after i rebooted my PC to change BOOT mode to UEFI Native, I was puzzled that my PC booted normally... GRUB UEFI, despite EFI variables error, was installed OK

I executed grub-install again and this time it went OK.

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