the firmware of the DVD Burner (this case your ND-3550a) controls the speeds you're able to burn on a certain brand/type of DVD. each disc is handled through it's own "strategy." for instance, i use Taiyo Yuden DVD+R 8x discs. with liggy and dee's firmware (v1.Y5), i can burn those TY discs at 2x, 4x, 6x, 8x, 12x, or 16x. with a different type of media, i might only be able to burn at 4x max because you won't get acceptable burn results if you burn any higher. i've seen a list somewhere (sorry, too lazy to search right now...) where it lists what media is compatible with the ND-3550a and at what speeds. you might find it on nec.com. the custom firmware you'll find in these forums will give you added write strategies so you might want to look that up as well.