How do I convert my linux disk from MBR to GPT with UEFI?

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:

• The author of gdisk has a verbose description of MBR, GPT, and UEFI.
• Clonezilla restore MBR disk to 4TB disk (convert to GPT) -- LINUX (not Windows!) covers the first part of the process.