I have an Apple at home that I've downloaded iTunes songs on and I've downloaded some on my PC at work via a usb keychain. When I transfer a song from one to the other, it says the original file cannot be found. I have to leave the usb keychain in the usb port for the PC to read it. I thought I could legally put a song up to five computers? How do I set it up where I don't have to use the usb and just load it on the PC harddrive?