Does the Pioneer drive show up under device manager or simply as one of the drives in My computer? If so, you're probably using old versions of Nero and Alcohol. See if upgrading both software help.
Or it could be wrong jumpers on the drives. How many HD/CD drives do you have in your system and how are they setup position and jumper wise? Assuming you have only 1 HD, set it as master and put it on IDE1, then set the Plextor as master and put it on IDE2, follow by the Pioneer as slave on IDE2.