Answer
This question was answered on March 13, 2005.
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LG Electronics - 4120B
DVD writer supported either the DVD-R/-RW format or DVD+R/+RW, dual format, supporting both Plus and Minus media.
NEC - ND2510A
The ND-2510A adds support for yet another type of media, DVD+R9. This is a variant on conventional DVD+R with two blank DVDs stuck one on top of another. This dual layer (sometimes called double layer) DVD media is the conventional 1.2mm thick, so each layer is 0.6mm thick, with the layers joined together with a special adhesive.