system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does formatting a 90% full 3TB hard drive instantly erase everything on it, yet deleting everything on it takes a few minutes?
gpt
Think of it like a huge book with a table of contents at the front telling you where everything is:

* **Formatting**: Ripping out the Table of contents and replacing it with a blank page
* **Deleting a File**: Carefully using white-out to remove an entry in the table of contents
* **"Shredding"** or **Securely Deleting** a file: like Deleting a file, but also replacing the actual pages for that section in the book with new, blank pages.

As you can see, formatting a drive doesn't matter how much of the drive is "full", and doesn't require much modification, so this is always quick.

Deleting a file is similarly quick, but still slower than formatting because you have to find the file's metadata (name, what folder it's in, where on the disk the actual data is, what time it was created) and carefully erase just that one record. The actual data is left alone.

Secure deletion involves not only deleting the metadata, but also writing over the actual file data on disk several times with random data. This takes the longest.

(Note, you may see options to "Quick" format or "Full" format - I've described a quick format above, a "Full" format is a quick format followed by writing zeroes to all the rest of the disk. This can take several hours on a 3TB drive.)