the purpose of disconnecting them is to see if you get anything displaying on your screen.
If the computer POSTS when they are disconnected, reconnect 1 of them, and try again, if it still posts, then it may be the other ones fault, its basic proccess of elimination.
If it still doesnt post with the HDD's disconnetced, you have to start pulling out stuff like RAM, CDROMS, Sound CARDS, MODEM, NIC, one by one, and see if it starts posting when you do it.