does the computer post (make the beep sound) when the plextor is on the pci card? does it by any chance make MORE than normal beeps in this configuration?
if it isn't even posting, I dont know that I would continue to try to go this route, they may be a battle not worth fighting.
if it does post, check your boot order options and make sure your seagate(i think this was your boot drive??) is listed first.
you don't have raid setup on the pci card do you? this should be jbod for using the plextor.