Can I replace optical drive with 2nd hard disk on MacBook?
I have heard good things about the Optibay kit. It provides a mounting bracket for whatever hard drive or SSD you want to put in there.

This video shows how to install it in a Mid-2010 MBP, which is close in design to the Unibody MacBook you have. Instructions according to the guy in the video:


Remove the bottom panel
Disconnect and remove the Wi-Fi/Bluetooth antenna.
Disconnect optical drive and put the connector onto the Optibay
Put your hard drive into the optibay
Follow the steps in reverse to reassemble your computer.


The dangerous parts are when you pop up the delicate connectors with, say, an iPod opening tool. So the Wi-Fi may not work afterwards, and you would be on your own since Apple doesn't support modifications. But overall, it isn't so bad. It looks like an easy fix.