Do magnets really damage your laptops?

I have using my laptops for quite some time now. I carried it to all my field work. Yesterday, as I put my laptop in a machine that has a big magnet on it, my colleague reminded me not to place my laptop besides the magnet. He claims that it will erased all the data on my laptop's hard disc drive. It's my first time to heard such thing, so I inquire some explanation from him. He said that since hard disc is being run thru magnetic fields inside it the magnetic field of a magnet will alter all the data inside the hard disc. Quite and interesting theory, so I move my laptop away from the magnet. Is this true? Do magnets really create damage on laptops?