Today probably every computer user know and used to get use of a recovery software to recover deleted files and folders. Those softwares are very useful when we delete some important files let's say you have a photograph that only in your PC and you've mistakenly deleted it. So then recovery software is like came from heaven. But on the other hand think you formatted your hard disk and you sold it out. And the photograph that you're bothered should be a very private one. And the guy who got your PC recover your hard disk from the same manner that you recovered your photograph. So he got your photographs and what do you think he will gonna do with it.
The most important thing with data is private and confidential which are deserved. So when we come to the point of data confidential, data recovery becomes very critical. So let's look how to remove a file forever and ever.
First we will look how files can be recovered. Actually any data on a disk / flash drive cannot be erase like you erase pencil marks on a book. When the data have written on the disk they last forever. Then you may ask "If files cannot be deleted what are we do by deleting files?". You may know File Allocation Table (FAT). (If you want to get some idea of file allocation table and what is file system means look What are File Systems). File Allocation Table has links to data on the disk with files names , attribs ,... So when we have deleted a file , actually its deleted from this FAT table. It's remove the link to the data on the disk from the label of the file. But on the disk the real data (ones and zeros) are exist and they will remain until we over write some data on top of it. The data recovery software is doing , scan for those data on the hard disk. Some times you may have seen when we scan hard / flash drive from a recovery software some file indicate as unrecoverable. Those are files which are over written by other data. When data is over written they cannot be recoverable. There is a phrase "Data cannot be deleted but can be over written".