Why does deleted data stay on a HDD once written, waiting to be overwritten, as opposed to being removed when requesting deletion?
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For almost all purposes, freeing up the space on the drive for reuse is good enough. As others have said, the way you do that is to simply take the entries out of the file table. Tiny fractions of a second to do versus minutes for large files. You secure the “deleted” data by securing the computer, and implicitly its drives. And if you can’t do that, perhaps you ought to ask why you’re putting data on it that needs to be totally removed?
If you REALLY want to overwrite files, you can either wipe the whole drive or use “eraser” software to do so at the file level.
Having said that – it’s worth pointing out that, for magnetic media and with the right forensic tools, even after a full wipe it’s still potentially possible to recover at least parts of the “deleted” data. (Like any physical mechanism, drive heads have a degree of tolerance in their operation, meaning that they can only be so accurate in their positioning – so simply overwriting a bit with a new one doesn’t guarantee that the old magnetic pattern is actually fully replaced.)
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