How do I wipe the hard drive of an old mac before I throw it away?

Use a drill. Make a few holes through the plates (the shallow dimension of the drive). If you can’t do that then use a hammer and a nail – it’ll just take longer. Judging from your username I’m guessing you’re in Japan so it should be no problem finding/borrowing powertools. Are you absolutely sure there is no Apple recycling program there?

Don’t wipe it, wash it!
Toto washer

April Fools

Why don’t u sell it? Download DBAN and burn it to a CD and then boot it up?
http://dban.sourceforge.net/
I have never actually used it, but I have read good things about it and have it burned to a CD in case I never need to use it.
Advantages of selling=earn money, most people u find to buy it wouldn’t steal your personal data or even know how to start.Plus, reusing is better than recylcing.

Magnets!

@delirium Magnets only work on cassette tapes!

@ delirium. Where would you get industrial strength/degauuser style magnets. a cd burner and an internet connection are much more common.

Okay, i’m from a family of scientists. I can get delightfully scary things fairly easily. I forget that most people can’t.

What is the feminine form of tool?

blender. will it blend? blendtec.com

dban is the way, as mentioned above.
Run it a few times overnight and even the NSA will have trouble cracking what used to be on the drive – then recycle it somewhere – good for the environment :)

Run your original OS X installer discs and do an ‘Erase & Install’. I believe there are options to “zero data”, which will overwrite the info on the drive with zeros. Then you’ll have a fresh install od OS X with no personal data and you can donate the Mac to Goodwill or some other charity that accepts old computers.