Take a look at your hard disk. From the lid, you should be able to make out the outline of where the disk is.

If in doubt, on the botton of the disk you should be able to see the motor, which directly drives the disk spindle.

Below the disk is the read/write head actuator spindle.

Note a point roughly half to three quarters of the way from the centre of the disk and its circumference. This is where you will be doing the damage!