Hacking a disk that's been 100% written to zero.
To get data off a disk that has been written to zero you need to hack at the hardware level.
When you write to a disk the head is not always exactly centred on the track. Sometimes it is off a bit to the left, sometimes off a bit to the right. So there may be a smear of previous contents as a weak noisy signal, if you are able to command the head to offset to various normally non-commandable positions left or right of nominal centre, and pick up the analogue signal from the head for nonstandard processing rather than feeding it into the standard disk-read signal processor code.
A three-letter agency might even have something like a large electron microscope to image the magnetic state of every square nanometer on a platter, and something like an image-processing system to decode it.
If you write multiple garbage patterns the chances of any off-centre data remaining goes down. I imagine each pass trashes about half of what was left by the previous one.
There is also whatever is left on the bad blocks that were replaced during the disk's lifetime. You have to assess what is the chance of a random four kilobytes written at a random time in the past being of any interest, and what might the consequences be? I'd hope that a disk's firmware erases a bad sector before relocating it, but unless the manufacturer specifies that it does you should assume the worst.
Biting the hand that feeds IT
