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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Think of magnetically written data as paint on your wall. If you no longer want a blue bedroom wall, can you remove the blue? It’s not quite impossible, I guess you could use paint thinner until all the blue is gone, but there’s no reasonable way of removing the color: all you can really do is paint over it.
Magnetically written data is effectively the same: you can’t really delete it, you can only write more data over it. Just like you could paint the wall white, you could make the new data all zeroes; but you’re still painting over it, not deleting.
So as deleting isn’t an option (for magnetic storage; optical and Flash work differently), you’re left with either simply overwriting later or overwriting now and then overwriting again later. The second takes extra time and adds wear and tear to your disk, for generally no advantage (if you need data security, disk encryption is a much better solution anyway).
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