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Convert Ubuntu from UEFI mode into Legacy mode.

Use this procedure only to convert an UEFI mode Ubuntu installation to boot in BIOS/CSM/legacy mode. Such a conversion may be necessary if some hardware doesn't work correctly under UEFI mode. (Graphics cards are a common source of problems.) Converting to boot in BIOS/CSM/legacy mode while Windows boots in UEFI mode can make the boot process more awkward -- you'll need to use the computer's built-in boot manager to switch between OSs, and some computer's have such poor boot managers that this may be impossible.

• If Ubuntu is installed on a GPT disk (you can check it via the sudo parted -l command), use GParted partition editor to create a BIOS-Boot partition (1MB, unformatted filesystem, bios_grub flag) at the start of its disk.

• Start Boot-Repair, and select Advanced options -> GRUB location tab.

• Uncheck the Separate /boot/efi partition option

  

• Click the Apply button in the lower right corner.

• Set up your BIOS so that it boots the HDD in Legacy mode. Generally this setting is located in the Boot tab → Boot order section of the BIOS.

_{Source: revised from: UEFI - Community Help Wiki}