What recovery techniques (if any) exist that can recover data from a mechanical HDD if the user has filled the disk with a single run of dd if=/dev/urandom of=/dev/[disk]?
Or in other words: is this method of "wiping" the disk secure? If not then why not?
It would be interesting to know if any purely software based approaches can rather trivially recover the data in this case, or if special recovery hardware is required (if it is at all possible).
How would this change if the user used dd if=/dev/null of=/dev/[disk] instead of urandom? In this case I believe special hardware can attempt to read the previous magnetic state of the platter to recover the data? What is this technique called?
How does this change with SSDs?