It might help you use some oil on the drill bit when drilling into the case.
Use a drill press,
fasten the hard drive to the table,
then place a spot of oil where the drill bit tip hits the outer cover
and proceed to drill away.
While drilling at a ssssllloooww (SLOW) RPM
continue to add a drop of oil every so often to help cool
and lubricate the drill tip.
I case you are using a hand drill, just be sure that the hard drive is secure before drilling. This will keep the hard drive from catching on the drill bit and spinning out of control.

I am sure that you are aware of the safety aspects of drilling, but I just wanted to highlight them in case others wanted to use this same idea in disposing of their hard drives.
