Ubuntu has really grown over the years and has become the best Linux OS today. If, for some reason, you need to always keep a copy of Ubuntu with you, you can create a bootable USB Flash drive for it.
It is better to use flash drive rather then CD as its not always possible to carry a CD safely with you. A CD can also get damaged easily and a few scratches will ruin some of the data on the disc which will in turn lead to improper working of the Live CD. So, a bootable USB flash drive for Ubuntu is the best option if you need to use live Ububtu frequently.
How To Create a Bootable Ubuntu USB Flash Drive From Live CD: If you already have a live cd of the latest version of Ubuntu then it will be very easy to create a Live USB Flash Drive.
Ubuntu has a built in application, “USB Startup Disk Creator” for this purpose. Unlike your normal Live CD this application will create a Ubuntu Live USB ‘Persistent’ Flash Drive. In a live CD all the changes you make are reverted back to the default Live CD settings after re-booting. But unlike the live CD, the Persistence feature saves all the changes you make in the USB drive even after rebooting.
Boot with the Live CD and Insert a USB flash drive of 1GB or more.
Now go to Systems > Administrator > USB Startup Disk Creator (on the top bar).
Select the inserted USB disk, then select Stored in reserved space option.and click on ‘Make Startup Disk’. Now all the required files will be copied to your USB flash drive.
That’s it. To boot from the Ubuntu Flash drive you just created, change the enable boot via external disk from the BIOS. Sometimes just enabling won’t work, you will have to set the highest boot priority for your USB device.
Note that I have tested this for Ubuntu 9. With the new versions of Ubuntu being released, you may have to use a USB flash drive with more space.