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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An HDD is like a library. The data is written in a huge number of “books” called blocks, but all the blocks are only identified by numbers. To find something, you need to use a big index to know which information is in which book.
Your computer uses the index to keep track of which books are being used at any given time. If you delete the index entry for some information, two things happen. One, the computer no longer has any way to find which books contain that information. Two, the computer now thinks that those books are empty. Of course, if you were just waltzing through the library and picked up one of those books at random, you’d find all that info is still there. At some point, part or all of it may actually get deleted because the computer, thinking that those books are empty, will write some new information there. But there’s really no guarantee on how long it would take to completely overwrite some deleted data through average drive use. To really delete stuff you can use software that will just write empty data to all the books in a file before deleting its index record.
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