Hi guys I have a 27" iMac which I have acquired from a friend, it currently has no hard drive in it, it boots up and chimes, fans spin , have connected it through thunderbolt to hdmi but nothing comes up on my tv...? The 3 led diagnostic lights are on so therefore video card seems to be communicating with the logic board , 4th light is not going as I currently have disconnected the LCD (faulty). My question is do you need a hard drive with os on it in order to use the thunderbolt ports?