Being able to backup / image to CD/DVD is apparently a glitch within Windows 7. Even Microsoft forums have documented these issues. Some of the documented issues involve the type of media used (+R, -R, +RW, -RW, dual vs single layer), the brand of media, the brand of burner, the speed of burning, whether the media is formatted separately prior to attempting the burn or during the burn process, etc. Windows 7 "Create a System Image" problems
From personal experience, the first few times I tried creating an image to DVD, I tried 3 different brands and two different types (a +R and a -R) because that's what I happened to have available. It flat out didn't work. I invested in an external USB hard drive and it has worked flawlessly ever since.