Seriously, BE CAREFUL! Hard drives contain all kinds of parts that can go flying, even if you're hitting it somewhat gently. Older hard drives can even contain glass.
You don't need a lot of force to get the drive open. Several lighter hits are better than fewer heavier swings (and safer, too). You just need it to separate enough so you can peel the top back.
When done, box up all components and turn them in at your local electronics recycling center.