Although I am not the OP, I was part of the thread from which I split this topic. What was being discussed were utilities that literally overwrite free space in order to either erase data entirely or corrupt it in such a way that it cannot be reconstructed.
It's entirely possible to rearrange the partitions on a hard drive that's been used but be able to run "undelete" or recovery utilities (think TestDisk/PhotoRec) on what's been newly created and reformatted in an effort to reconstruct and recover data. Under Windows utilities like CCleaner's Drive Wiper feature can be used to obliterate data in free space (or even an entire drive) by overwriting it one or more times (1, 3, 7, or 35 overwrite passes). There are others that do something similar.
He's asking does something exist under Linux that will do a "block by block" overwrite of free space or an entire disk to ensure that the data on it is no longer accessible by recovery utilities.
Brian AKA Bri the Tech Guy (website in my user profile) - Windows 10 Home, 64-Bit, Version 1809, Build 17763
Presenting the willfully ignorant with facts is the very definition of casting pearls before swine.
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