You probably read about it in Cryptonomicon (I know that's where I read about it), but I don't know if Stephenson made it up or not.

As to practically It seems quite unfeasible. First of all since magnetic forces fall off according to the inverse square law, you're going to need a seriously large magnet to for it work at say 1 meters distance or so. You're basically going to have seriously retrofit your entire house, and it's going to be very hard to hide. Secondly and more importantly even the most powerful commercial hard drive degaussers require that the drive be in contact with the magnet for up to 10 seconds to guarantee that all the data is erased, so someone just walking through a door isn't going to be in the field for anywhere near enough time.

Unless you can get your hands on a powerful magnetic monopole, the field falls off with the inverse CUBE of distance. (Note: noone has so far managed to get their hands on a magnetic monopole, powerful or otherwise.)

It can still fall with the inverse of the distance by smartly assembling several dipoles.

Anyway, the time issue remains.

I guess I was even more right than I thought I was :)

As others have noted: Cryptonomicon.

...but that basic idea did give me another one: electromagnets on a separate circuit inside the drive enclosure or laptop body.

Fail to pass a security check on schedule and the circuit is engaged, destroying the HDD.

I'm no hardware wizz although I did make a motor out of a nail and copper wire once.

Well, you could just use a solenoid to actuate and make lots of metal contacts. Or you could use some sort of iron filing injection system that litters iron everyone in the computer.

The problem then is false positives. Only 1 mistake and kablooie.

As a student, I visited the High Field Laboratory in Dresden (Germany). They have one of the stongest magnets in the world, I think it was 80-100 Tesla, and supposedly stronger than a similar one in Los Alamos (although the one in Dresden could only be powered up once, and would vaporize immediately). The whole setup was surrounded by ~3m thick concrete walls.

And right next to the magnetic coil, behind a small blast shield, was an old PC... an early 90s beige box looking like a 486 or early pentium, controlling some measurement equipment.

We asked how that worked, and whether there are any problems with "EMP" with the hardware or the hard disk. The answer was, no, the PC works fine. But you have to be extremely careful not to leave any screws or screwdrivers around, because "above xxx T/m field gradient, iron starts to fly", as evidenced by lots of scratches on the concrete.

That's also part of the fiction book "Cryptonomicon" by Neal Stephenson.

Looking at conventional hard drives we see pretty strong magnets a few CM away from the platters so you'd need something freaking huge to wipe the drives passing theough a doorway.

You'd practically need an MRI machine to get any real effect.

I'm pretty sure I saw this on one of the CSI tv shows.

Not the case with SSD.