How to boot an Ubuntu ISO without a thumb drive or a CD. Read more to find out.

I was wanting to try out an older version of Ubuntu on my netbook; but I did not want to write the ISO image to a thumb drive to boot it; therefore I copied the ISO image to a subfolder of my /boot folder; /boot/iso and then I added this section to my /etc/grub.d/40_custom file to enable me to boot a live Linux distribution ISO from the hard disk instead of the hassle of booting from a USB thumb drive and all that jazz.

exec tail -n +3 $0

# This file provides an easy way to add custom menu entries.  Simply type the
# menu entries you want to add after this comment.  Be careful not to change
# the 'exec tail' line above.

menuentry "Ubuntu 10.10 ISO" {
	set isofile="/boot/iso/ubuntu-10.10-desktop-i386.iso"
	loopback loop (hd0,1)$isofile
	linux (loop)/casper/vmlinuz boot=casper iso-scan/filename=$isofile noprompt noeject
	initrd (loop)/casper/initrd.lz

I am doing this with Linux Mint 12 Debian edition and this is working perfectly. A good trick if you have downloaded an ISO on your laptop or netbook or even a desktop PC and you want to try it out before writing the ISO to anything. Just fill out the file above like I have and then run this command.

sudo update-grub

This will update the Grub bootloader and add the custom boot menu that will allow booting the ISO on your machine.