I had a cd-r disc with just a few pictures on it, and i wanted to burn a 18 picture slide show on the same disc using dell picture studio. When i put the disc in the computer said "disc contains data please insert blanc disc". Is there a way to overide this to get more on the disc?
Afraid not. Once you computer closes the disc, it will always read as though it is full. This is why I use cd-rw for stuff like that. They can't hold as much information, but you can erase them, or add more data whenever you want. And they only cost a little more then a cd-r.
Indeed, CD-R is WORM (Write Once Read Many) technology and thus can only be written to once.
CD-RWs don't necessarily hold less data though. Average data storage space for them now is 700MB (80min), same as average CD-R. I've only got a few 650MB ones, the rest are all 700s.