hey guys, i have itunes on both my macbook, and my PC, half of my songs are on each one... but if i load my ipod with say my songs on my mac, then go to put the ones on the PC onto my ipod, the ones on the PC REPLACE the ones on my mac, instead of ADD. why does this happen? do i have to have all of my songs on one computer? or is the a way to solve this? thanks guys R.