I Do not think the Burn app can rewrite to a DVD-RW disc once it is written, but not sure as I don't regularly use OS X Burn app.
If you use a burning application like Roxio Toast, I believe it has the ability to rewrite over DVD -RWs with no issues.
I THINK YOU'RE RIGHT. WHAT I FOUND I COULD DO WAS TO USE THE BURN PROGRAM, AND SELECT ALL THE ITEMS ON THE DISK AND ANY ADDITION ITEMS I WANT TO ADD. THE PROGRAM THEN ERASES AND REWRITES THE ENTIRE DISK. ALTHOUGH THIS TAKES A LITTLE MORE TIME THAN JUST BEING ABLE TO ADD OR SUBTRACT INDIVIDUAL FILES, IT HAS THE ADVANTAGE OF UPDATING FOLDERS OR FILES THAT MAY HAVE CHANGED.