Create a usb boot drive with support for legacy BIOS and 32/64bit UEFI in a single partition on Linux
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README.md

How to create a 3-in-1 bootable usb drive on Linux.

A usb drive with only 1 partition to load grub2 on usb-bootable machines with Legacy BIOS, 64bit UEFI or 32bit UEFI.

Partition the drive and install grub2

Warning: the usb drive will be formatted, save your data before proceeding!

First of all, on you current installation, check if the folder /usr/lib/grub/ exists and is not empty. If it is empty or does not exist, make sure the package grub-common (or equivalent for your distribution) version 2 or higher is installed. Depending on the system, /usr/lib/grub/ will contain one or more of the following folders: x86_64-efi, x86_64-efi-signed, i386-pc, i386-efi, ...

The x86_64-efi, i386-pc and i386-efi folders need to be present in order to install the corresponding bootloader on the usb drive.

Install them using the package manager, for instance on Ubuntu:

sudo apt install grub-pc-bin grub-efi-ia32-bin grub-efi-amd64-bin

Now, find the device file for your usb drive. Here, the file is /dev/sdX. Replace X with the appropriate lower case letter(s) in the commands.

Make sure it's the right drive! (check the capacity and the partitions) :

sudo fdisk -l /dev/sdX

Open fdisk :

sudo fdisk /dev/sdX

Press the following keys (THIS WILL ERASE ALL DATA FROM THE SELECTED DRIVE!) ... :

o <enter>

n <enter>

p <enter>

1 <enter>

<enter>

<enter>

t <enter>

e f <enter>

a <enter>

w <enter>

... in order to :

  1. Create a new empty DOS partition table
  2. Create a new partition
  3. Select primary partition type
  4. Set partition number to 1
  5. Start partition at the first possible sector
  6. Set partition end to the last possible sector
  7. Change partition type
  8. Set partition type to EFI (FAT-12/16/32)
  9. Enable the bootable flag on partition 1
  10. Write the partition table
Create a fresh filesystem in the newly created partition :

sudo mkfs.fat -F32 /dev/sdX1

Mount the filesystem :

sudo mount -o umask=000 /dev/sdX1 /mnt

Write the MBR and install the grub files required for legacy BIOS boot on the drive :

sudo grub-install --no-floppy --boot-directory=/mnt/boot --target=i386-pc /dev/sdX

Install /EFI/BOOT/BOOTX64.EFI and other grub files required to load grub from a 64-bit UEFI firmware :

sudo grub-install --removable --boot-directory=/mnt/boot --efi-directory=/mnt --target=x86_64-efi /dev/sdX

Install /EFI/BOOT/BOOTIA32.EFI and other grub files required for 32-bit UEFI :

sudo grub-install --removable --boot-directory=/mnt/boot --efi-directory=/mnt --target=i386-efi /dev/sdX

Create an empty file /.disk/info to avoid issues with some grub versions :

mkdir /mnt/.disk

touch /mnt/.disk/info

Create a grub.cfg file :

touch /mnt/boot/grub/grub.cfg

Example grub.cfg with Xubuntu 18.04 Live

(skip this if you already have a working grub.cfg for the usb drive)

Create a folder for cd images :

mkdir /mnt/isos

Create a folder for the OS files :

mkdir /mnt/isos/xubuntu18_04-i386

Download an Ubuntu cd image (for example: Xubuntu 18.04 32-bit) :

Note: make sure there is enough space on the usb drive.

wget --directory-prefix=/mnt/isos/xubuntu18_04-i386 http://cdimages.ubuntu.com/xubuntu/releases/18.04.1/release/xubuntu-18.04-desktop-i386.iso

Extract vmlinuz and initrd from the iso archive :

isoinfo -i /mnt/isos/xubuntu18_04-i386/*.iso -x "/casper/vmlinuz.;1" > /mnt/isos/xubuntu18_04-i386/vmlinuz

isoinfo -i /mnt/isos/xubuntu18_04-i386/*.iso -x "/casper/initrd.lz;1" > /mnt/isos/xubuntu18_04-i386/initrd.lz

Edit grub.cfg :

nano /mnt/boot/grub/grub.cfg

Write or paste something like this :
menuentry 'Xubuntu 18.04 i386'{
	search --set=root --file /isos/xubuntu18_04-i386/vmlinuz
	linux /isos/xubuntu18_04-i386/vmlinuz locale=fr_FR console-setup/layoutcode=fr boot=casper iso-scan/filename=/isos/xubuntu18_04-i386/xubuntu-18.04-desktop-i386.iso quiet --
	initrd /isos/xubuntu18_04-i386/initrd.lz
}

Notes :

  • The search command on the second line is only useful if you install the bootloader and the OS files on different partitions.
  • Remove or change the value of the locale parameter to set the language of the live system.
  • Remove or change the value of the console-setup/layoutcode parameter to change the keyboard layout.
Save grub.cfg (in nano) :

CTRL+O

<enter>

CTRL+X

Finish

Unmount the filesystem :

sudo umount /mnt

...