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My laptop have already installed Windows 10 in UEFI but I mistakenly dual boot Ubuntu OS In legacy mode. So everytime I have to change BIOS SETUP settings to UEFI for Windows and legacy and legacy first for Ubuntu. So what I do to when I start my pc it ask me which OS to use(it also ask currently but don't run the other one according to BIOS setting)

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The switch from legacy boot to UEFI boot in Ubuntu should be fairly easy, only a few steps are needed.

First of all make sure that Windows is fully shut down, Windows Fast-startup-feature should be disabled. Boot your current Ubuntu installation.

You need to know which partition is your EFI-System-Partition (ESP). You can use the command lsblk -f to get the device name and UUID of the ESP. The ESP is formatted in FAT and normally has a size of a few hundred MB, mostly the partition will be labeled and you can find it easily.

Create a directory as mount-point for the ESP with

sudo mkdir /boot/efi

Now add a line to /etc/fstab with the following content:

UUID=XXXXXXXXX   /boot/efi  vfat   umask=0077    0    1

Use the UUID you found before.

Mount the ESP with sudo mount-a

Now install the grub-efi-amd64-package:

sudo apt install grub-efi-amd64

This will automatically remove grub-pc (the legacy boot-mode version of Grub).

Other commands like grub-install or update-grub are not needed, installing grub-efi-amd64 will install the boot-loader to the ESP during installation of the package.You are ready to reboot.

Enter the UEFI-settings and check your boot-order. I'd also recommend to disable CSM in UEFI-settings to speed up the boot process.

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  • I don't understand the downvote, I've successfully converted my legacy install to efi this way. – mook765 Dec 1 '20 at 13:33
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I suggest you to reinstall Ubuntu in UEFI mode cause moving from legacy to UEFI not so simple. You should:

  • Boot from Live-USB;
  • Create FAT32 partition (minimum 33MB of size) and mark it with boot flag;
  • Enter chroot mode by:
    • sudo mount -o /dev/sdxX /mnt where xX is you drive and partition number where a root partition is;
    • sudo mount /dev/sdxX /mnt/boot/efi
    • for i in /dev /dev/pts /proc /run /sys; do sudo mount -B $i /mnt$i; done
    • sudo chroot /mnt
    • for i in /dev /dev/pts /proc /run /sys; do sudo mount -B $i /mnt$i; done
    • sudo chroot /mnt

Now install grub:

apt-get install --reinstall grub-efi-amd64
grub-install --recheck /dev/sdX

or just:

dpkg-reconfigure grub-efi-amd64

It should create 'ubuntu' menu item in UEFI.

Now edit your fstab by sudo nano /etc/fstab and add the following to it:

UUID=bla-bla  /boot/efi  vfat  umask=0077 0  1

You should run blkid to identify which UUID your efi partition has.

Check which packages are installed by dpkg --get-selections | grep grub

grub-common                                     install
grub-efi                                        install
grub-efi-amd64                                  install
grub-efi-amd64-bin                              install
grub2-common                                    install

Those are packages should be installed for UEFI boot. All other you should remove.

Also check that /EFI/ubuntu/grubx64.efi file is exist.

Exit from chroot by Ctrl+D. Unmount all in /mnt and try to reboot.

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