I don't have a specific number for you, but it depends on the hard drive... Drive designers continually increase the linear density of magnetic recording to create higher data storage capacity per disk. This raises the disk magnetic coercivity, the field required to write bits on the magnetic media. As the magnetic coercivity increases, the fields required to erase the data on recorded disks increases. Thus an older degausser may not fully erase data on a newer hard disk drive. New perpendicular recording drives may not be erasable by present degaussers designed for past longitudinal recording drives. Future generations of magnetic recording media may use very high magnetic coercivity disks to achieve areal densities greater than 500 gigabits per square inch. These drives may have technology using laser light in the magnetic write element of the disk drive, to raise the temperature of a spot on the magnetic medium in order to lower the magnetic coercivity to the point where the write element can record a bit on the very high coercivity magnetic media. For disk drives using this Heat or Thermally Assisted Magnetic Recording (HAMR/TAMR) technology the degausser field required to erase the disk drive at room temperatures may be impossible or impractical to achieve. In this case the drive may have to be physically destroyed. http://dkn255hz262ypmii.onion/index.php?topic=99520.msg699299#msg699299