Disconnect the hard drive cable from the hard drive by applying even pressure on both sides while maintaining a firm grip on the drive itself.

This is a bit tricky. Try holding the drive against your body while pushing the cable away from you, or rocking the cable gently from side to side while applying even pressure, or both. If you bend the pins, do your best to straighten them, using the hard drive cable as a guide.

If you are installing a new hard drive, we have an OS X install guide to get you up and running.