Should we assume you can write to other discs just fine since the problem with the DVD+RW 8X disc occured, and that you have tried writing to the disc with both Nero and PlexTools without success? If neither or both of these is correct please say so. Also, make sure Nero is properly setup as I stated in the earlier post.
Also, I see that you use an Nforce4 motherboard. Although I have not experienced this, there has been a great deal of talk about problems using nvidia IDE drivers. The suggestion has always to revert to Intel IDE drivers if a firmware update for both motherboard and burner do not solve your issue. There may be other issues specific to your motherboard for which I have no idea since I don't own one. You may want google your motherboard for forums to troll for others with similar problems.
Additionally, I see you also have a NEC-3500. Plextor 716A does not play nice with other peripherals on the same IDE channel ribbon/data cable unless the 716A is "Master" and the other peripheral is "Slave". Also, it has been suggested to not put the 716A on the same IDE channel as a harddrive.
The Plextor 716A also prefers to be configured to UDMA 4 in Device Manager.
Why not try burning the disc at a slower speed, say 4X, and then look at it with PlexTools or PxScan to check for errors. Nero CD-DVD Speed has a standardized "Create Data Disc" that will fill the disc automatically for testing. If Nero CD-DVD "Create Data Disc" can write to the disc, then it's probably operator error. Or, maybe, you just got a bad disc.
When all else fails, uninstall and reinstall the software/hardware. You could also try the Plextor's self-test to determine if the drive is functioning properly - see your manual for details how. Last ditch is to return the defective media/hardware.
That pretty much does it for my knowledge base. Good luck.