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If you want to try out a new Linux distro, be it the latest Ubuntu 11.04 Natty Narwhal daily ISO or any other (I've only tested it with Ubuntu though!) and don't want to burn a CD each time you want to try a new daily build (and you don't have an USB memory stick around), you can use a cool GRUB 2 feature that lets you boot a live CD ISO directly from your hard disk. You can also use this method to boot varous utilities such as Super Grub Disk, SystemRescueCD, Parted Magic and so on.

Usually, setting up GRUB 2 to boot an ISO can be quite a difficult process (well, not that difficult but for some reason it always failed for me) but you can do this with just a few clicks thanks to Unetbootin. This info is actually available on the Unetbootin website but I somehow missed it - until now (thanks to Usemos Linux).

I've recorded a video showing the Unetbootin Grub2 menu entry and then booting the latest Ubuntu 11.04 daily ISO using this method, however the video is very bad in quality (my cam sucks) so I didn't embed it into this post, but you can watch it @ YouTube if you want.

Install Unetbootin and set everything up for booting an ISO using Grub2

Warning: remember, when messing with Grub (or any application that modifies it), there are things that might go wrong so use this at your own risk and if you know how to fix it in case things go wrong.

1. Install Unetbootin:

In Ubuntu, simply run the following command to install it:
sudo apt-get install unetbootin

If you're not using Ubuntu and Unetbootin is not in your distro official repositories, download it from HERE.

Important: like Scirocco points out in a comment, there are issues with some Unetbootin versions which not only fail to work when trying to boot an ISO from the hard disk, but will also fail when you try to create a bootable USB. I've used Unetbootin 494 from GetDeb and everything worked fine so I suggest you use the same version. So if you're getting an error like this: "disk image not found. need to load kernel first", it's because of the Unetbootin version you're using.

2. Using Unetbootin to create a new Grub2 entry to boot the ISO:

In Unetbootin, select the "Diskimage" option, then browse for an ISO and under "Type" select "Hard Disk" and leave "/" for the "Drive", then click OK:

Unetbootin boot Grub2 ISO

Once Unetbootin finishes, restart your computer and select "Unetbootin" in the GRUB2 menu. If the Grub2 menu doesn't show up, press and hold the SHIFT key.

Because Unetbootin extracts the ISO, you'll have to run it again when you want to boot a new ISO, even if it has the same name and is placed in the same location as the old one.

Removing the ISO from GRUB2 menu

Unetbootin remove ISO grub2

The next time you'll launch Unetbootin, it will ask you the following: "Unetbootin is currently installed. Remove the existing version?" - this isn't about removing Unetbootin but about removing the Grub2 menu entry for Unetbootin (and the ISO). Select "OK" and the ISO will be removed from the menu.

Update: if using Unetbootin doesn't work for you, you can try the manual instructions Dan Sonny Radu posted in a comment below - it's not as easy as using Unetbootin though.

Update 2: reader payet07 mentioned in a comment below that this works with Burg too. "But with some themes (i.e gnome) no icons appears. You just have to change for another theme at boot (i.e ubuntu2) to see see it right". However, he used the manual method for settings this up.

Many thanks to Usemos Linux for the heads up!