Oleg Petrovsky of HP posted a good article on hard disk firmware hacking on the HP Security Research Blog. Long post, lots of pictures, very informative!

“In light of the recent publicity around malware that can remain persistent in hard drive firmware, it seems reasonable to seek a better understanding of what actually happens inside the hard drive – specifically, an understanding of the firmware. […]”

http://h30499.www3.hp.com/t5/HP-Security-Research-Blog/HDD-firmware-Hacking-in-the-dark/ba-p/6780246