Question: Companies try to zero-fill their hard drives several times when they decommission them, why? Surely one pass of zero-fill would do the job exactly as they need. I understand the reasoning for the companies to do it but the need to run it several times. Could someone explain the process to help me with my understanding.
Answer: Physics. Rewriting the magnetic domains on the platter weakens them, but still leaves a residual amount of magnetism. The level of magnetism drops below the threshold to be readable by the electronics in the drive, but it is easily detectable with the appropriate equipment.