grub2 usb

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> # Create a new empty DOS partition table

n <enter> # Create a new partition

p <enter> # Select primary partition type

1 <enter> # Set partition number to 1

<enter> # Start partition at the first possible sector (default)

<enter> # Set partition end to the last possible sector (default)

t <enter> # Change partition type

e f <enter> # Set partition type to EFI (FAT-12/16/32)

a <enter> # Enable the bootable flag on partition 1

w <enter> # 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 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