Think of it like a pencil and paper.
When you erase something on a computer it’s kind of like erasing something you wrote in pencil. There is still a little trace of what you wrote on the paper even though the graphite is mostly gone.
It’s almost exactly the same with a hard drive.
This is why when you delete something people can basically “trace over it” and get the file back, though if might be slightly messed up in some ways.
It can get harder to see under if you write something new over the old data and delete that too. If you do that enough times it can get really hard to see, but just like with paper it will eventually wear out the hard drive and instead of a clean white piece of paper, you basically have a grey dirty one. At that point writing on it doesn’t work so well.
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