But I have no idea how practical is this.
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.
Anyway, the time issue remains.
...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.
The problem then is false positives. Only 1 mistake and kablooie.
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.
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.