Just for grins and giggles, download ImgBurn, put a dual layer disk in the drive and start the program.
On the right side of the upper window, Imgburn will show the disk information…if you have put in a Verbatim DL disk, it will tell you the disk id code, which will either be MKM001 or MKM003 depending on which speed you bought. There will be a lot of other information about the disk also.
If it can identify the disk correctly, it should burn to it correctly as well.
Have you actually tried a burn with Roxio?