Are you sure it's not playing the copies of the music on the smartphone, rather than the copies on your computer. Even if you copy over the music, you could still be playing the copies on your smartphone. Hope that makes sense. What type of computer are you on?
If its downloaded you may need to click on the icon lower right corner and turn this device off and choose remove device. Sometimes with windows machines by simply unplugging the cord will mess up the system causing this turn off or the machine to freeze up.