An active terminator is a device that connects to the cable Rather then me try to explane it go
here to get more info on this and some of your other questions.
To select what device ID number you want on the hard drive there is a space located beside the 50 pin cable that alows you to place a jumper over two pins telling the controller what ID you want to use if there are none then it is ID 0. if you place the jumper on the first two pins (look at the face of the drive there should be a diagram of what pins are what) You will select ID 1 and so on.
