My macbook does not mount or read blank DVD's

My mac won't read or mount my blank dvd+R. I have tried all 25 of them they are new. I've checked my firmware and found that it should be ok. I was able to burn one of them but it will not read the rest, I've been at it for days. It reads my regular dvd's for movies but not the blank ones. I thought I should transfer my photos and videos to these blank dvd's because my mac keeps telling me my start up disk is full and it has become so slow, applications hang, and I could no longer upload any more photos to it. Please let me know how I can resolve these problems, with the dvd's and my startup disk. Please help!