backing up specific bulk data

Excuse my lack of technical jargon and my tendency to sound like an idiot, computers are not my forte.
I am backing up all of the data on my ancient macbook (OS X 10.6.8) and would like to be able to put all of the music saved to the computer's hard drive onto an external hard drive. A lot of the music is already on the external hard drive, but I don't know exactly what. Without going and visually recording what is already on the external drive (there are literally thousands of songs), is there a way to only send the music that is NOT on the external hard drive - in bulk - from iTunes onto the external hard drive?
Thanks for any help.

Gday and welcome to the forums.

There is no easy way to do this. I'm not sure what Mac and what OS you are running, but if you have all these songs in 1 folder (not sure if you are using itunes or not either), but if they are in 1 folder, you should be able to select all in Folder 1 you want to move, and drag them into the other Folder 2. When this happens, the songs will start to copy over, until you reach one that is already there. In this case, a dialogue will come up telling you there is already one of these in the Folder, what do you want to do, "Keep Both" "Replace" and a checkbox with Apply To all. Check the checkbox and click Replace . . .
This should then have all your music in one Folder. . .

If these are going into iTunes, drag the whole of Folder 1 into iTunes, let them copy in, then goto View>Show Duplicate Items, and delete every second one. . . Or use a Duplicate Finder App like Gemini or one of many Duplicate Finders in the Mac App Store.

macbook OS X 10.6.8
I'm not trying to get the music into itunes. i am trying to get the music files from the computers hard drive (the files are in iTunes already as well as in a folder on the desktop) onto an external hard drive. The issue is, a lot of the songs are already on the external hard drive and I don't want to manually compare what is already on the external hard drive.
Is there a way to select a large batch and have only the ones not already on the external hard drive transferred?
Sorry if the original post wasn't clear, but I'm pretty sure I stated all of this in it.

simplyput, simply put, "no" (sorry, couldn't resist)

You can write something to do the compare, or you can look for a third party product to do that, but in the generic OS X, the copy function won't do that.

There is a third party product, ChronoSync, that will sync your internal and external devices and you can select what folders/files to sync, but that isn't free. Carbon Copy Cloner and Super Duper+ will let you copy all the files from the internal to the external just like Copy, but with the ability to schedule it on a regular basis.

BTW, if the files were copied into iTunes, you don't need two copies of them. Just back up the iTunes copies periodically and you should be good to go.