I shall get rid of that duplicate thread for you.
Which version of Windows are you using?
Mastered is Microsoft's name for Disc At Once. So you put files into the queue, tell Windows to burn the files, and at the end of the process, Windows should close the disc.
The Live File System option is the one that's supposed to support adding/removing files without erasing a disc, because it only closes the session (not the entire disc).
If you can still add/remove files from the disc after you actually burn the disc, and you chose the "Mastered" option, then...there may be a bug? Or operator error, which is all too easy?