I have iTunes on my windows PC that I used for my iPhone prior to buying my MBP. I now have content on both machines. My iphone is set to manually manage the content, yet when I tried to put content on my iphone from the MBP it wouldn't let me. I did an erase and sync, which cleared the iphone (didn't lose too much) and I could now put the content from the MBP on. Now I've gone to my PC and cannot add content from that onto my iphone. How do I enable this? I could do it with my ipod from a few years back.