Instead of the Memorex (??) disks, you may have better burns using Verbatim DVD+R DL instead. If you want to you can save the Nero output to the HDD and burn the disk using ImgBurn - which is reckoned a better burning software (and it's free).
I don't have Nero, but isn't it at the step just before the burn that you select the burner or the HDD.