Did you re-boot?
I think I got this one once after the system had been on for, well weeks, and I was probably trying to cure cancer on something strange and unusual on the computer the hour prior. Usually a reboot helps if the foregoing doesn't.
As to the disks, I also know that I got this problem when I didn't know if the disk had been written on
Now I try to keep my blank media on the spindle and not take them off until I need to burn.
Also look at what you have loading when you first turn on your computer. I think I saw somewhere that if a program that helps you burn to a disk, like "In CD" by Nero, is loaded on the system that can cause conflicts.
One last thing on that note, if you bought the plextor drive and you installed their software (whatever software that is now) for writing to the plextor, remove it. This can cause conflicts too. Especially if you got an older plextor drive discounted with the original software. The plexwriter software of old doesn't work in XP. At least not very well.
Hope this helps, but I would agree with the admin, need more information. Just stabbing in the dark now.