Overwriting free disk space
Overwriting free disk space leads to erasing earlier deleted files from selected disks.
Depending on the amount of free space, the number of passes and some other factors, overwriting can take a substantial amount of time.
It should be borne in mind that even after the complete erasure of all the file data, including the file system entry about it, in the directory (folder) file in which the erased file was before the removal, may be it (erased file) name, if the directory also has not been removed.
Due to the nature of specific storage devices and software, we can't always guarantee rewriting of all 100% of the selected area. For example, magnetic disks may develop new "bad sectors" after data have been written. Modern hard disks often feature automatic remapping of marginal sectors or tracks, which the OS may not even be aware of. Attempts to counter data remanence by overwriting may not be successful in such situations, as data remnants may persist in such nominally inaccessible areas.