The device-diagnostics company Blancco claims to have bought over 100 second-hand hard drives on the eBay sites of 4 different counties, to see what data, if any, could be accessed on them. It claims that 42% of these drives still stored information from their previous owners that can be thought of as sensitive.
https://www.notebookcheck.net/A-data-security-company-claims-that-up-to-42-of-used-hard-drives-sold-on-eBay-still-have-important-data-on-them.419669.0.html
How many passes of 0s or 1s do you need to overwrite? Is 1 time enough? What about SSD?
Do total drive overwrites work or not?