Dragging songs from an external library on to an I phone.

I recently transfered my itunes library to an external hard drive. When hold option while opening itunes and select my external library, it opens just fine. I tunes shows the songs on the external library and I can play them on my computer. However, now it wont let me drag songs from the library onto my iphone. The little plus sign doesn't show up like it used to. Does anyone know how to fix this?