I've been working on PCs for over 15 years, and I can tell you that magnetic screwdrivers CAN and DO erase hard drives. Worse yet is the damage they can do to the chipsets on motherboards, controller cards, etc. I once shorted out an entire motherboard because of a magnetic screwdriver. They teach you this in the A+ certification courses. A magnet like those found in magnetic screwdrivers is actually several times more powerful than the magnetic surface of a hard drive, and it is positively charged. When it comes near enough to the surface of a hard disk to cause a charge, the data that is stored on the disk can get scrambled beyond recognition. Remember, it's being held there by a magnetic surface as well, but a weaker one.
Some chipsets include what is called firmware - just like software, except that it is coded right into the chip, so the chip knows its own function. This can also get seriously jumbled up by a magnetic screwdriver.
This is why actual PC repair kits come standard with non-magnetic screwdrivers. I bought a whole set of non-magnetic just for the purpose of working on computers.
- Ken LePage