Re: Why did my disk not erase?

To erase magnetic media the field needs to be very strong near the 
surface. Magnetic field strength reduces with the square of the distance, 
so in your case, the field generator simply couldn't generate a strong 
enough field in the disc surface. Disk housings are usually aluminium, 
which would not have any significant shielding effect. Disk heads fly a 
few thousandths of an inch from the surface, so a relatively small field 
form the head can magnetize the disk media. A magnet, say, a few inches 
away, i.e. a thousand times the distance, would need to be 1000 squared 
times as strong, i.e. a million times, to achieve the same field strength.