This is taken from the following URL: https://www.raspberrypi.org/documentation/hardware/raspberrypi/bootmodes/msd.md
</color red></color>
I have documented it here as I use this guide a lot, and I don't want to lose it :)

  This tutorial explains how to boot your Raspberry Pi 3 from a USB mass storage device such as a 
  flash drive or USB hard disk. Be warned that this feature is experimental and may not work with
  all USB mass storage devices. 
  
  <color red>USB Hard Drives seem particularly fussy.</color>

Before a Pi will network boot, it needs to be booted with a config option to enable USB boot mode. Enabling this config option requires special </color red>start.elf</color> and </color red>bootcode.bin</color> files. These can be installed by using the “next” branch on </color red>rpi-update</color>.

sudo mkdir /mnt/target sudo mount /dev/sda2 /mnt/target/ sudo mkdir /mnt/target/boot sudo mount /dev/sda1 /mnt/target/boot/ sudo apt-get update; sudo apt-get install rsync sudo rsync -ax –progress / /boot /mnt/target

cd /mnt/target sudo mount –bind /dev dev sudo mount –bind /sys sys sudo mount –bind /proc proc sudo chroot /mnt/target rm /etc/ssh/ssh_host* dpkg-reconfigure openssh-server exit sudo umount dev sudo umount sys sudo umount proc

sudo sed -i “s,root=/dev/mmcblk0p2,root=/dev/sda2,” /mnt/target/boot/cmdline.txt sudo sed -i “s,/dev/mmcblk0p,/dev/sda,” /mnt/target/etc/fstab

cd ~ sudo umount /mnt/target/boot sudo umount /mnt/target # sudo poweroff