The problem is that i wanted to chage external disc drives and I had this when I plugged in the new drive. I discovered that Disc Utility was the cause. I has checked my old drive with this and because i had not changed to home drive on exiting it was still referencing it. I plugged in my old drive and opened disc utility. I then highlighted the home drive before exiting.I unplugged my old drive and I restarted the computer . After startup I plugged in my new drive and opened itunes and tried to burn a disc andit worked as normal.... phew!!!