Hi XP_Office.
Do you mean that you want to backup your 500GB drive to your 120GB drive?
It really depends on how full your 500GB drive is and what type of files they are. You could use some desktop compression utility (zip, tar etc) or a program like Symantec Ghost. Both options will basically create a single file with the entire contents of your 500GB drive. I will warn that this is more for archival purposes and if you're regally accessing this data neither technique is ideal.