system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If deleted files on your computer are still there (how the authorities catch people with illegal content), is possible to eventually download and then delete enough data to keep your hard drive full, even after deleting everything?
gpt
Yes-ish.

What happens is the data isn't destroyed, the space is marked as "available" and can be written over again. Think of a hard drive like a bunch of boxes. You put files in those boxes. When you delete a file, you're actually marking the box as "this stuff can be replaced" but you haven't actually removed it from the box yet, and you won't unless you have something new to put there. It's possible to have things in all the boxes but still have available space. Simple recovery software can "look" in those boxes.

There is software that can intentionally "0-out" a drive to re-write either the whole drive, or the available space on the drive. There are Department of Defense certifications for secure erasure that not only 0's it out but then writes random data, and repeats a certain number of times. (Secure erasure should include writing random data, because even if writing all 0s traditional hard drives are magnetic and very careful analysis can reconstruct which bits were changed - although this level of recovery can cost thousands of dollar to *attempt*.)

Addendum: Keep reading below if you're interested, apparently modern hard drives don't strictly need multiple passes.