|It is possible that your computer has JUST enough power (power supply) to power what it has, but even a second hard drive is too much for the power supply (unlikely but possible).|
OR if you can get into your BIOS with both hard drives installed, check your boot order to make sure the correct drive is listed in boot order and the other is not (or lower down if you will need that option later).
OR it is possible that the 2nd hard drive has a short in it and that is causing the problem, the only way to be sure is to try it on another computer. If it is new, return it for replacement.
You have to be a little bit crazy to keep you from going insane.