if you see a file with the iso extension on your burned disc, you did it wrong. when you burn, make sure you are burning an disc image! (ie, the iso contains a copy of the filesystem of the original dvd, when you burn this iso back to disc, you need to burn the iso as an image. that recreates the original filesystem. if you put the iso ‘on’ the disc, then you are just creating a data dvd, with the iso file on it.

in img burn this option is just called ‘burn iso to disc’, but in many other cd/dvd burning programs it is called writing an image to disc.

are you sure this is what you did? sorry you keep making so many coasters :p
http://forum.imgburn.com/index.php?/topic/6380-how-to-copy-a-disc-using-imgburn/