When you burn on to a CDR (even if you only put a small amount of data on it) the burning program will write over the entire disc. This is because a table of contents is written onto the disc and this cannot be amended later. When you put a disc with any burned data in the CDRW drive attempting to insert more you will get a message saying the disc cannot be written onto any further. There's no way round this for CDRs. CDRWs however can be overwritten and re-used but again not added to after disc has been burned.