A hard drive is divided into segments - maybe I can find a picture. When you write a file to the drive it's written to one or more segments. There's an index area on the disk which associates the file name with a specific segment. From there indexing schemes vary but it all works to tell you the segments the file uses and in what order.

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When you delete a file to save time all that happens is the index entry for the first segment is labeled as not being used, OK to write over. Now you can delete a file with something like the infamous Bleach Bit that Hillary used. That overwrites the whole file, bit by bit.

So if you just delete you can search the index to find the first segment, which will lead you to the next segment, etc. This was a not uncommon chore in the floppy disk days, sometimes things just go wrong. The file entry in the index started with the file name. If the file was deleted the first character of the file name was replaced with a null. Not too hard to find and fix. Beyond that if you find one segment it'll tell you where the next segment is and depending on the file system maybe where the previous segment is.

Now the Bleach Bit erase is harder. If you overwrite an entire file bit by bit you can still read a ghost image of the old file with the proper equipment. Bleach Bit and others overwrite the file a number of times with a specific pattern. This is specified by DOD for erasing secret files. Pretty much makes data recovery impossible - why Hillary isn't staying at the old graybar.

( I use the shred utility that comes with Free AVG. I trust they've implemented the DOD specification properly. But there's been nothing on my computer that would warrant a difficult and very expensive retrieval if AVG didn't get it exactly right.)

The key elements in human thinking are not numbers but labels of fuzzy sets. -- L. Zadeh

Which explains a lot.