That means you are trying to burn 6 GB of data to a DVD that doesn't have enough free space. For that image size, you will need a Dual Layer disc. I recommend using Verbatim DVD+R DL 8.5 GB. This disc is more than enough to write that amount of data. After you acquire this DVD, insert it and then add the data. Make sure to choose a low writing speed to increase reliability in the future and that's it.