NEC 3520a general concerns....

I am another newby to this recording stuff, so please forgive me if this is redundent. I have a 3520a NEC DVD recorder in my computer (windows XP with 2 gigs of ram and an AMD 3200). I use my NEC to copy my DVD's and CD's that i use for my business. I have this version since november and have probably recorded at least 400 copy's on my DVD and 250 CD's. I hand these DVD's and CD's out at my seminars. For the most part CD's seem to perform perfectly. But on occasion I get reports that some of my clients have problems playing my DVD's . I have found that the Minus DVD's seem to work best so I use Minus. I am not sure why some people is so....but I have several questions. First of all how long will this NEC continue to produce DVD's that seem to be reliable and coherent? The cost of these NEC units have gone down dramatically so replacing it is not a problem. I would hate to copy a bunch of DVD's, that I would later find to be defective. Also is there any way to clean or service this unit to help it become more reliable and to extend it's life time? I use NERO and just use the normal way to Copy. I have not learned how to down load this to my hard drive and then record it from there. But I am afraid that if I keep copying one disc to another, I may inadvertantly copy a defective disc and that would then become my norm. Perhaps I should learn how to copy this to my harddrive. I have heard to get maximum efficiency that one should only copy a DVD at 6 X and a CD at 24 X. If anyone has some ideas on these issues I would very much apreciate it. Thanks in advance for your time.