Have a look at the pic of the rear of the 1640, below:
The jumper settings are to the left of the IDE pins. If you look closely at the picture, you'll notice that the drive is currently set as slave; the jumper is actually occupying the centre two pins. If you want to set it as MASTER, then you need a get yourself a set of pliers (or something that will remove the jumper easily) and delicately pull the jumper out and move it across to the right set of pins. It's a very easy, no-brainer, procedure.