I just installed a Western Digital Black Scorpio 750 GB Harddrive using a (noname) Optibay, alongside my Vertex 3 SSD. Unfortunately the WD is very noisy compared to the slower HD i used before. Actually its a real relief once it spins down. I will probably swap it with another drive soon. Until then theres a way to make this a bit more acceptable by setting a shorter delay for the spindown of the drive. Apple default seems to be around 10 Minutes, which was way to long for me.
it is possible to set the spindown time in minutes. It can also be disabled by setting to 0.
Since i dont need permanent access to the HDD this helped a lot!