nondaj

I am not only new to this program but also to the art of burning discs. I have a DVD+R disc in the DVD drive. I burned one folder of files. When I returned to burn another group of files, I got a message that told me I need to insert a new disc. I have lots of room on the inserted disc in the drive so would like to use it. What have I done in error to get this message and what can I do to avoid losing space on these discs when I burn to them?