I've posted extensively on this issue, as I went thru 2 bad drives, and a hellish experience w/Plextor U.S.A. Bottom line for me, no more Plextor, which is unfortunate, because Plextor drives do things other drives don't.
Anyway, if you haven't already been told, run the Plextor hardware diagnostic. Your manual tells you how. Without looking it up, as I recall you have to disconnect the IDE cable, do something w/the jumper (can't exactly remember, but manual has a picture), put a disk in the drive, hold down the eject button while powering up the system, and wait while it runs thru an internal diagnostic. At the end, the LED light will blink green, either one blink or two blinks, over and over, w/a short pause in between blinks.
Again, please do read the manual, as I am recalling the diagnostic steps from memory--you don't want to hose up the proper sequence.
I forget what one blink means, but both my bad drives had two blinks, which means "initialization error".
My drives started out great, then gradually began to produce errors in reads and writes--small ones at first, then more and more until the drive finally refused to even read a disk, much less write one. This is the pattern I have seen reported here, and on other forums.
I am happy for the people who managed to get good 716A's that are still working, and I wish I was one of them. Tho, having now replaced that drive w/an NEC 3520A, and added another drive--an LG GSA-4163B--I have most of what I had w/the Plextor... for a total cost of less than what the one Plextor cost. :Z