Why is my computer resistant to magnets?

I wanted to quickly & conveniently erase all the data on my computer, so I rubbed a powerfull magnet around the computer, and all my data is intacted!

It's no biggie, but does any have an explanation why it didn't trash any data?
My computer is only 2 inches thick; the magnet can grab things from about six inches away, so I know the magnet should have done it's job.

Well it depends on how much shielding the computer has. If you wanna trash the data on the computer, just run a burn program on the hard drive. like this one: http://dban.sourceforge.net/