To erase a drive you need a high frequency AC magnetic field. Bringing a powerful magnet close to a drive will "corrupt" the data as the individual flux lines will change certain bits. The drive may be unreadable, but can be reformatted and possibly repaired. A hammer is a better erase device.
And I know of where I speak. A few years ago I had the opportunity to acquire the magnet out of an IBM 370 Hard drive unit (circa 1980). This was one of the 18", 6 platter drives with a linear motor. The linear motor magnet actually weighs twenty pounds and is about 8X8X4 inches. A real beast. As a test I placed a VHS tape close. Still played. I then put the tape on top of the magnet and let it sit. It still played. I then investigated how magnetic tape, and subsequently hard drives and computer media worked. And what it takes to erase them. High frequency, or rewriting the bits.
BTW, it will totally screw up a CRT TV from 20 feet away. This is one bad magnet. AlNiCo.
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