Bootable USB

This is how I installed grub unto my external hard drive (GPT) which had Ubuntu 17.04 installed. When you do this it allows you to boot your Ubuntu on any PC which supports UEFI by using the grub on the external without relying on the PC grub. It can also serve as backup grub in case the is problem with EFI of Windows.

Installing Grub to an external hard drive that has Ubuntu installed

1. Boot into an Ubuntu live USB and connect the external hard drive.

2. List partitions of all devices: lsblk

3. From the results of lsblk, identify the linux partition and also the efi partition of the external hard drive, i.e. /dev/sdXY1 and /dev/sdXY2 respectively. Replace XY1 and XY2 with your own partition names.

4. Special mount the linux partition:

   sudo mount /dev/sdXY1 /mnt

5. Mount critical virtual filesystems:

   for i in /dev /dev/pts /proc /sys /run; do sudo mount -B $i /mnt$i; done

6. chroot into the Linux partition you mounted:

   sudo chroot /mnt

   You are now in the external hard drive's linux filesystem.

7. Create the directory where grub would install its files:

   mkdir -p /boot/efi

   If it already exists, then skip to step 8.

8. Mount the efi partition from step 3:

   mount /dev/sdXY2 /boot/efi

9. Install grub to the external hard drive:

   grub-install /dev/sdX

10. Update grub:

    update-grub

11. Find the UUID of the efi partition (aka 'vfat') and note it down:

    blkid

    or

    ls -l /dev/disk/by-uuid

12. Now we need to tell fstab to mount that efi partition on boot:

              cp /etc/fstab /etc/fstab.bak
              sudo nano /etc/fstab  
            

    Add the below two lines to fstab replacing xxxx-xxxx with the UUID from step 11:

              #my modified fstab to mount external hard drive's esp  
              UUID=xxxx-xxxx   /boot/efi   vfat   umask=0077   0   1  
            

    Make sure to comment out the fstab entry of the Windows esp so it doesn't conflict.

13. Exit the chroot:

    exit

14. Reboot the PC:

    sudo reboot

At this point when you reboot and choose your external device from the EFI boot manager, it will boot to grub.

To make Ubuntu automatically boot when you insert the external and Windows to automatically boot when the external is not inserted you would have to create a custom boot entry for Ubuntu to point to the efi on the external drive:

Creating a custom boot entry for Ubuntu to point to efi on the external HD

From Windows EasyUEFI can add or remove boot entries.

From Linux efibootmgr can add or remove boot entries.

Alternative #1 – using EasyUEFI

1. In Windows download EasyUEFI, install it and run.

2. Choose the EFI options manager

3. Choose create 'new entry' with + sign

4. Choose Linux or other OS and give it a name in the description box

5. Select the FAT32 EFI partition on the external drive that contains the grub bootloader

6. Choose browse

7. Navigate to /efi/ubuntu/shimx64.efi or /efi/ubuntu/grubx64

8. Save

9. Move it to the top of the boot entries list

10. Restart

Alternative #2 – using efibootmgr

1. List boot menu entries:

   efibootmgr

2. Create a boot entry:

   efibootmgr -c [-L label] [-d /dev/sdX]

   e.g.: efibootmgr -c -L myubuntu -d /dev/sdX

   This boot entry would automatically be the first entry.

   /dev/sdX is the external hard drive with the grub bootloader.

3. You can now reboot and without your intervention Ubuntu would automatically boot when external hard drive is plugged in. When the external hard drive is not plugged in, the PC would skip our Ubuntu boot entry to the next entry which is probably Windows Boot Manager.