Seattle-light

How do I know if I'm using dual layer media? Would it be obviously marked on the media itself? (Sorry if that's a dumb question).

This is not a problem per se. When I create an image file from a DVD in ImgBurn, I get an MDS file and an ISO file. I'm just wondering which file I should select when I want to write the image back to a DVD. Will I be successful with either one? Is one a more ideal option than the other? I understand that the MDS file is (I think) sort of a pointer to the ISO file, but is it necessary? Thanks!