it depends on what you want to backup
the installation, your personal files or everything?
usually when you need to do a backup of the installation mylivecd is a app that will create a livecd with your configs and apps you have there and afik you can choose to add or not other files but this will affect the size of the iso
if you want to do a backup of your personal files sometimes is just copy and paste the files or folders you want from the hard disk to the external storage or use a specific app doing this for you, there is one in pclinux control center and other more in the repos but never tried them
if you want to backup everything there is apps capable of do this but i never used them really, only did some experiments with clonezilla livecd but never tried to restore, the installation never failed