I am trying to burn photo files to a dvd that already has some photo files on it.

I am trying trying to burn photo files to a dvd that already has photo files on it, for backup. There is plenty of room on the dvd for additional files. It seems I need a BLANK dvd to burn the files. Has anyone else found a way around this?

I have Macbook pro with Yosemite, using Photos software. I export the photo files to a file on my desktop, then right clip and burn those files to a dvd. no problem. Then when I want to add more photo files to that DVD, I get a pop up box that says insert a blank DVD into the burner. There is still plenty of room for additional photos on the DVD. Can anyone help, please? Thanks