(no subject)

I have a question for the internets. The whole disappearing music saga has lead me to conclude that I need a better backup plan than the current "dump it on the external hard drive whenever I think about it." At this point, I have things on the external hard drive that are not on my computer and vice versa. I'd like a way to transfer the files on my computer which are not on my external hard drive, without deleting the files which are currently on the external hard drive. The obvious solution to this would appear to be a program of some sort that would scan both drives and move only the things which are not present on the external hard drive, but it's not at all clear to me that the programs I'm seeing on basic google search do this. The wording tends to be more along the lines of cloning your computer's hard drive to the external and deleting files that are not on the computer's hard drive.

I'll freely admit that I could be reading this wrong and, if so, please correct me. What do you use to back up your hard drive? Do you use your external for storage for things that don't fit on your desktop? How do you avoid having multiple copies of the same file on your external hard drive?