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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Computers have something called pointers, it basically points to where something is stored. Kinda like a file shortcut.
To read a file from the hardrive, the reader needs to be pointed to the file by a pointer.
If any part of the hard-drive isn’t being pointed to, its considered empty.
So if you have a certain file on a hard drive, and want to delete it to recover the memory, you could take time overwriting all parts of the drive to have 0s, or you can simply delete the pointer to that part of the drive, skipping the partially lengthy process.
Next tome you want to use that space, the system will see it as empty and just overwrite it.
If you later want to recover that data though, its still written there and potentially accessible.
If you really want to delete things securely though, you need to overwrite garbage values over it multiple times, as they do leave partial trace of their old states, but that’s really only for super top secret stuff, at which point you might as well destroy the drive afterwards just in case.
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