I don't know what happened but all of a sudden itunes won't let me drag and drop a song onto my ipod or iphone, It won't even let me drag and drop a song in a playlists within itunes. Does anyone know what the problem is?
I'm having the same problem and I already restarted everything. Still doesn't solve the problem.
My solution was to right click on the song / album cover / video and select "copy", than select a playlist or the iPhone music list and click "paste" using the iTunes menu on the top (Menu>Edit>Paste).