1 Download Ubuntu Desktop Live or boot-repair-disk
2 Create a bootable USB from the downloaded iso file using balenaEtcher or Ventoy suggested, refer to the following guides if necessary
List of free software/programs/Utilities to write ISO files to USB HDD/Thumb Drive/Flash Drive (Create bootable USB drives) easily etc. (balenaEtcher)
Another Simple and open source Multiboot USB creation tool (Ventoy)
3 Configure the BIOS/UEFI from the system which we want to convert the disk, so that we can boot it from USB (Refer to the motherboard manual or Search the motherboard model if necessary)
(If Boot Repair is asking to update the software, click on Nobutton)
4 Launch the terminal from the live system
5 We use the following command to convert the disk from MBR to GPT
(If gdisk is not installed, use sudo apt install gdisk to install it first)
sudo apt gdisk /dev/sdx
Note: Replace /dev/sdx with the correct disk label e.g. /dev/sda, /dev/sdb etc.
Output
GPT fdisk (gdisk) version 1.0.5
Partition table scan:
MBR: MBR only
BSD: not present
APM: not present
GPT: not present
***************************************************************
Found invalid GPT and valid MBR; converting MBR to GPT format
in memory. THIS OPERATION IS POTENTIALLY DESTRUCTIVE! Exit by
typing 'q' if you don't want to convert your MBR partitions
to GPT format!
***************************************************************
Command (? for help):MBR to GPT
Enter w to convert MBR to GPT partition Enter y to confirm and write the change
GPT to MBR
Enter r to enter the recover and transformation Enter g to convert GPT to MBR partition
We can enter p to check the partitions.
Warning: We will lose our bootloader like grub, which means we will have to reinstall/fix the bootloader/grub etc. after the conversion, we can use boot-repair-disk from Step 1 to fix it easily by following steps prompted by the software