...because it will NOT do anything to your data.
Did you see what is inside of normal Hard Disk? Very strong magnets! I always recover them them from failed drives - they are strong as (whatever you believe: hell or anything).
About 20 years ago we took floppies (5??", double-sided, 360kb capacity, and 1.2Mb) to the Aluminium Smelter plant - where people with pacemakers are prohibited to enter. After few hours of moving aroud the source of heavy magnetic field (busbars carrying ??1,600 kA (1,600,000 amperes of DC current - if you need this clarification). The data was readable without problem after our experiment.
About 15 years ago driving small Suzuki SUV across magnetic field created by over 3,000kA flowing through busbars over 4 m away (behind reinforced floor ) was stalling the engine, but did NOT affect data on electronic devices (PSION) and various magnetic disks.
If you extend 5m tape measure it stick to aluminium smelting pots and stay horizontal without help; if you drop the spanner, dont look for it on the grond - it is stuck under the equipment. Such strong magnetic field you won't be able to recreate in your "home" lab.
This days strong magnet does not affect even the screen - as it used to with CRT displays.
BTW: Do you know that CRT's were made for specific hemisphere (north, south) because of compensation of Earth magnetic field?
Once I got 20" monitor which the technician could not repair (colours were in wrong places, smudged etc).
After letting him sweat for few hours (arrogant type, deserving to get the lesson) I told him that HE won't be able to do it, but I can do it in 2 seconds without any tools. That statement made him more angry - adding some more time to his lesson. When he was "cooked enough" I turned the monitor up side down. Everything became perfect. After few minutes of silence he asked very quietly and politely: "what the F...?". I explained to him that the monitor was probably destined for NORTHEN HEMISPHERE and compensated for the magnetic field there, not in South Africa!
Don't underestimate Earth's magnetic field and overestimate man-made ones. Steady, constant (i.e. non-alternating) magnetic field won't wipe data.
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