And additionnaly, isn’t there a way to exploit this so we can store more stuff on PCs?
Edit: can’t thank you all individually but thanks to everyone, I learnt something today, appreciate all of your replies!
Because you’re operating system is lying to you, for efficiency sake.
Imagine an old school library, books on the shelves, and a Dewey decimal card catalog index in the center.
You want to delete a book, to make room for future books, so you tell the librarian delete this book. And she removes the card from the card catalog index, and turns to you and says the book is gone!
In this scenario the book is still on the shelf, but the index no longer points to it.
Clearly the book isn’t gone, but from your perspective you don’t have to wait for the book to disappear, and the librarian knows eventually she’s going to clean the shelf, and remove whatever isn’t in the index.
That’s more or less, with a lot of hand wave in, what operating systems do for file systems.
In this analogy, when you add a new book, only then is that “deleted” book actually removed and replaced with the new one. Until then, it just sits there waiting, but since nothing is pointing to it, it’s hard to find.
When someone recovers a file, what they’re doing is going book by book and reconciling the index to see if there’s anything missing. Since this book still exists, it can be recovered.
To extend this a little further, computers also don’t actually store books, they store blocks.
For example, you have a computer that can store 50 blocks of information. You store “Moby Dick”, taking up 20 blocks & “Tom Sawyer”, taking another 20 blocks.
Next you decide you don’t like “Moby Dick”, so you delete it. You also decide you want to store an ice cream menu, taking up just 1 block.
That menu will be stored based on where the computer thinks the block fits best. So you might have 20 blocks that still contain “Moby Dick”, or you might have only 19 blocks that contain most of “Moby Dick”, but it might be missing the beginning, middle or end.
If I were doing data recovery I might not be able to provide you with the complete “Moby Dick” story. I might only be able to give you part of it.
Looking into why blocks, let’s say you’re writing up the first draft of a book report, it might take up 4 blocks. Then later you edit, improve and add to that that book report, and now it takes 5 blocks. The computer took care of making space, even though your report got larger. It didn’t know if you were going to add 1 new block of information, or 1000 new blocks of information, it figured it out and did the rearranging for you.
However when it comes time for you to look at it, it automatically knows how to put it together. (And usually it does group things together if it can).
This is important to keep in mind when it comes to data recovery because the more you use your computer the more likely blocks are allocated and data gets moved around.
If you delete important photos, then spend the weekend surfing the Internet, those photos might be gone. Or if they are available, might only be partially available.
Because of how filesystems work. There’s basically an index that tells the OS what files are stored where on the disk. The quickest way of deletion simply removes the entry in that table. The data is still there, though. So a data recovery program would read the entire disk and try to rebuild the file allocation table or whatever by detecting the beginning and ends of files. This worked better on mechanical drives than SSDs.
Yup, and many security suites will include a tool that writes all 0s or garbage to those sectors so the data can’t be recovered as easily (you really need multiple passes for it to be gone for good).
You have a notebook. On the first page, you put a table of contents. As you fill in pages, you note them down in the table of contents at the start.
When you want to delete a page, instead of erasing the whole page now (there are hundreds free still, why waste the effort), you erase the entry in the table of contents.
Now if someone finds your notebook, according to the table of contents there is no file at page X. But if they were to look through every single page, they would be able to find the page eventually.
This is loosely how file systems work. You can’t really use it to boost storage, the number of pages is finite, and if you need to write a new page, anything not listed in the contents is fair game to be overwritten.
If you remember the VCR days, imagine your hard drive is a copy of Bambi. You, in preparation for a family event need a tape to store footage of the event on. You decided that you haven’t watched or wanted to watch Bambi in a long time so you designate that tape as the one you’re gonna use when the party day comes.
At this point your hard drive (the copy of Bambi) has been designated as useable space for new data to be written in the future.
Bambi is not lost yet and wont be until you write to that tape, therefore if you wanted to you could watch Bambi in the time between now and the party even though you plan to overwrite it. Once Bambi is overwritten, its no longer recoverable but the interim between now when you designate it as useable space and when the space is used, the data persists.
it’s inefficient to really erase the data, so what happens usually is: it gets marked as deleted. the data only gets overwritten when another file is written in the same data area, which often doesn’t happen immediately. even if a drive gets formatted the empty metadata structures of the new partitions and file systems are just written on top. since they have no file entries yet, the previous data just sits there invisible and inaccessible until new files are created and maybe overwrite a bit of the old data.
