External hard-drive windows/macbook?

I have a WD external hard drive, I have a '09 macbook and a lenovo. I formatted the hard drive to be backward compatible between the two laptops. The issue i'm having is with Itunes. I copied all my music from an old windows and dropped it into the macbook. No problem there. I bought a new laptop, put windows 7 on it and now when i use the same hard drive the music is not transfering properly. Half the songs won't play on Itunes. HOWEVER if i just play the song off windows player there's no issue. Someone help please!