My Nec 2510 is refusing to read DVd9’s. What did I forget to do?
As far is a know, you don’t have to do anything.
Is it working perfectly under other conditions? Some people rapports that the NEC 25XX is a sub-par reader, although I don’t any problems what so ever.
My suggestions is to try some more discs to find out if it is a general problem or only with the disc in question. If it is a general problem, RMA the drive, as it is supposed to read DVD9.
Sounds like a faulty drive… so far, my 2510 reads all of my DVD9 collection without fail… 
Just an idea, and I may be wrong I often am! Have you installed the software to play DVDs such as WinDVD?