It's not the magnetic screwdriver that damages the hard drives. A hard drive can be exposed to the field of a VHS tape bulk eraser and survive unscathed.
Like Lyndon said though, static is a dangerous enemy. If the screwdriver picks up a charge from you and you get to close to the wrong chip or solder point -- ZAP!
The numbers I've heard regarding static were 300v for hardware, and 1500 to register on a nerve ending. Either way, if you feel it the computer felt it first.
Grounding is important, so either wear a grounding strap on your wrist or keep part of the arm that's inside the machine grounded against the case. *DON'T* use your other hand/arm as a ground. If there's current still available it will travel across your chest and kill you. One of the founders of the search engine that loads on the side of sSnakeSs died of electrocution while fixing his computer.