Send it to "jutizdepartment zürich".. :
No just kidding.
A long time ago I've read the magnetical, spinning hard disk drive needed to be overwritten 7 times to erase the side of the magetic tracks on the plates.
Later, but it's really way down in my memory, heard that 8 to 9 times was better, safer.
It's with a highly expensive equipment you'll find in some physics lab you can read the side of the tracks.
circumspect about the field of your employer? If you're a big fan of audio and loudspeakers, I'll hire you, will lauch a startup in this field in 2023..
:)
Regards.
--
Philippe Strauss