Hi and Welcome!
I presume that both (your CD and Plextor drives) are on same IDE channel? It would be better if you switch them (set Plextor as master and CD as slave), because faster drive (or in this case drive that needs better priority - PX716a) should always be set as master. Even if you leave them as they are, they probably wont cause you any problems....but if you can/know how to....then rather change them. In case you dont know how to...just open the case...and take both manuals if you have them (or just look at the back of the drives and change those pin settings....usually it reads there MASTER and SLAVE...or MA an SL...and also...usually...i.e...if you want to set drive as master...just close those 2 pins with that connector...otherwise...close slave's 2 pins....or...just come back and we'll help you out further)...
Guys at your PC shop probably didnt "have time" to change those settings...i believe that if you had only cd before...it was logically that it was set as master....and now...they only added Plextor...and set it to slave....rather then set it to master and set CD to slave....it was their lack of time, will or logic....
I believe others here will help you with their thoughts too....so be patient and wait a while....but as i said...they should be switched....at least i would do so....and majority of other probably too....your choice to make....
Good luck and enjoy!
SiliconFreak.
EDIT : Here is something I just found about your issue....
Adding an Extra Drive to an Older PC
One Cable - Two Drives/ Defining the Master and Slave Relationship
Unfortunately, this process often requires a bit more technology than merely plugging in. Older PCs use parallel ATA technology - 2 drives share one cable (known as a channel, and most PCs come with at least two IDE channels for a maximum of 4 drives. Setting a jumper designates each drive as either a master or a slave, which permits a single cable to connect two drives to one IDE channel. The jumper settings for each designation are usually labeled on the drive itself. A few simple rules should guide your configuration choices. If possible, each drive should sit on its own IDE channel configured as a master drive. If you have two drives on one channel always make the faster drive the master drive. For example, suppose that you wanted to add a second hard drive and a DVD burner to a PC equipped with one hard drive and one CD-RW drive. IN that case, you would want to set the new ,f aster hard drive as master on the primary IDE channel. Your older hard drive should be the slave drive on the primary channel, with the two optical drives as master and slave on the secondary channel. So to sum it up....faster drive always a master...in your case...(if they are both on same IDE channel...)...Plextor master, CD drive slave...