I think I found your solution. I have been working on a friend's Toshiba P25-S520 with the Pioneer DVR-K12D. She was having problem with Windows Media Player detecting the drive to burn music.
I was reading in another forum about several people with the same problem as you. This is what they did to fix it:
Go into device manager and remove the Primary and Secondary channel for IDE.
Remove the Controller.
Reboot so Windows detects them.
Reboot again so Windows can enable them.
(link to site where I found this.. http://hardware.mcse.ms/message101839.html
posted by antonis333)
I did this and now WMP detects the drive. Did not try to burn with Nero, but when WMP burned it did not seem like it was burning at 4x. The speed was more like 16x.
Hope this helps.