Sorry for my bad english. I had the same trouble with my 716A (VIA chipset KT400). It work fine now, with UDMA4 only on the first IDE channel in slave position after HD UDMA 5. All my others tests succeeded to PIO mode:
alone master, second IDE channel => PIO
master with slave, second IDE channel=> PIO
slave, second IDE channel => PIO
cable select (either master or slave on flat cable) => PIO
Extract from Plextor documentation, chapter 3: "We recommanded that you connect the DVD recorder as MASTER (factory setting) to the secondary E-IDE connector and that you do not connect a second device to this connector"
It doesn't work for me.