Below are methods for copying everything from one computer to another. If you have some things on one and other things on another and want to integrate them, it gets really messy and you need to use more complex methods.
iTunes: How to move your music to a new computer [or another drive] - http://support.apple.com/kb/HT4527
do not confuse moving your whole folder and library with moving just media files as in
iTunes for Mac: Moving your iTunes Media folder - http://support.apple.com/kb/HT1449
Quick answer if you let iTunes manage your music: Copy the whole iTunes folder and all its subfolders and files intact to the other drive. If you already have an iTunes folder on that drive, you can replace it when copying as long as you don't care about the contents.
Macworld - How to transfer iTunes libraries between PC and Mac - http://www.macworld.com/article/46248/2005/08/shiftitunes.html
I have almost zero experience with synching and can't help with that.