Hi, you should have chosen to burn the disc as a multi-session disc. You will not be able to burn any more to that disc. A multi-session disc allows you to place ever more items on it until it is full, it is then finalised.
Surely an 80 minute disc is now useless to record any extra music, just because I already have one song on it? yes basicaly thats it if you burn 1 song to a cd and thats the only thing you burn you wont be able to burn another song later unless you use a CDR cd rewritable and chose the mulltisession mode if you burner software has one its best to burn say 80 minutes of songs in one go to a cd