Open it and remove the platters. You can also remove the PCB to destroy the chips too just in case. With all the spy bugs on devices now days it's best to play it safe. The larger IC chips are most likely memory such as cache which you'll want to destroy. I'd imagine burning with a torch should do. You can destroy the rest of the chassis to make sure there arn't any other chips but really it's the platters that are the most important to destroy.
For the platters, I like to just scratch the hell out of them with a sharp object. You can fasten them to a grinder to do this faster, using sand paper. You can also pass them along low grid sand paper. Keep in mind that an actual line of data is VERY tiny, so even all the scratches the sand paper is doing is probably missing lot of data, but it is making it extremly difficult to recover, and traditional disk recovery methods in a clean room are out the window when the platters are scratched this badly. The NSA probably has electron scaners they can just put the platters on though so don't count on just destroying the heads. Also keep in mind that it is very hard to corrolate the data between each platter so just removing the platers and handling them already made anyone's jobs very hard.
I have a bunch of platters in a pile that are more than likely unreadable as is, but when I accumulate more I'll probably melt them all with some thermite. Mostly just for fun.
Also hard drives have big magnets in them, you can probably pass this magnet over the platters, pretty sure this will erase all the data on it's own.
I had actually built a rig that sended a small EMP to the platters, I would just stack them in the rig and throw the switch. I'd hear the power supply complain a little but but that's all she wrote. I stopped using the rig though because I did not really have a way to actually certify that it even did anything. That's probably why professional rigs for this are very expensive.
Long story short, take platters out, scratch the hell out of them with sand paper. Maybe drill a couple holes and bend them, and I doubt anyone is getting that data. You can even cut them in pieces and dispose of them in different garbages at different months.
What you could also do is all of the above, but put it all back together after, and fill it with spray foam just as a big FU to the government.
