[deleted]

Not perse. In that case you're erasing/reusing the ledger entry, but the 'new' file may get written on a wholly different location depending on how the OS works. But yes, it CAN work the same, if the new file overwrites the exact same spot as the old file. Most OSses work that way.

Many drive cleaner tools will have a tool to wipe free space. They write over every bit that is not occupied with 'undeleted' data or stuff you're using. They replace everything not in use with new junk data that could just be 00000001 for every bit. This takes a fair bit of time but doing this a couple of times can remove any trace of what a specific bit says or even used to say.

If you save somethinn as 'file.name' and it's contents are 00000111 then delete it, it stays as 00000111, but if you delete it then wipe free space it changes that 00000111 to something useless.

No, even if it's the same filename, there's no guarantee that it will be at the same place. Also, if your new file is smaller, for example an empty file, it can only overwrite a tiny part of it if ever it's at the same place.

SADs are much harder (perhaps impossible) to securely erase, you’re right

It’s worth thinking of computer memory less like a resource that’s used up, like water being scooped out of a bucket and drank, and more like sand in a sandbox.

Sometimes it’s being used for something meaningful, like being a castle or making the walls of a big hole or having letters written in it, and sometimes it just lying around doing nothing, but there’s not any more or less of it.

You might draw a line around a bit and say, “my castle is here, don’t use anything inside the line”, and at the end of the day you rub the line out and go, “I’m all done now, so do whatever”. Maybe you come back and the castle is still there or maybe someone has knocked it down and built something else with it.

Yep, that is what it means.

Exactly. The "free" memory is just the available memory, not the empty memory.
In fact memory that has been overwritten can still be recovered.
That's why there is software that "clean" your disk if you want, and it takes a while because basically what it does is just overwrite over and over again the disk until recovery is almost undoable.

Yup.

Also, fun fact, computers lie to users all the time. Eg, say you're playing a game and save. The game with very high likelihood will tell you it has saved well before the computer actually did it, and resume playing.

Why? Because people hate waiting, and the computer can actually do the writing to disk while you keep playing without making you wait for everything to be done.

There's many, many such optimizations.

People often ask why deleting files is faster than creating files. The answer is because the bulk of the file's data is NOT erased, because that's considered wasted effort. When the new file comes along it will write over the data, completely unaware of what was once there. So actually blanking out the old data doesn't really accomplish anything and the "delete" is really fast by just dealing with the file listing paperwork.

...unless there's privacy concerns that warrant actually clearing the data, specifically because preventing undelete from being possible is the goal. This is sometimes called "secure delete". But if you're uninstalling a video game, who cares?

Think of it as a table of content in a book. So when you "delete' a file, you just remove the table of content chapter marker, but not the pages. Then, when you need to write more data, you "write" to the pages then update the table of content.

Things can get more complicated when it comes to flash storage (ssd/flash drive) but only when it comes to overwriting the pages.

Imagine a binder with a table of contents, inside the binder there are pages, when deleting a file you just erase the page number, the page is still in the binder but you no longer know where to look. (If you want to find the pages again, there is special software, akin to looking through the binder page by page)

Some documents are several pages long, so when you later write something to the binder and replaces the pages, you might have page 4,7,9 of your document left, thats when it cannot be recovered anymore.