Folder Sync program?

I've searched around and I am looking for a program that will sync folders to my external hard drive. Essentially I want to take my iTunes folder and echo it to an external when it's connected which also updates/deletes content based on what I've done to it on the local drive.

I have SyncToy but it only seems to have the folders when it is connected to the computer that I echoed it from.

For example if I take my hard drive to a different computer the folder and the sub folders are no longer visible or accessible.

What am I doing wrong, or is there a better way to do this?

I assume you are running Windows. If you used Linux, I would recommend Rsync. So easy...

Good luck.

Found the issue was, for some reason, a Mac doesn't recognize music I sync'd from my computer?

Your disk is probably formatted for NTFS.