How do you find the data that has been "completely" deleted?

Q: Why do some "completely" deleted data can still be found back to recover it?

A: Some friends will be surprised: I clearly deleted the data, and emptied the Recycle Bin, why can I find it back? This has to start with the principle of recording data from the hard disk: When we put the file to the hard disk, the operating system first in the hard disk file allocation table to write the file name, size, And according to the free space of the data area in the file allocation table continue to write the contents of the file in the beginning of the data area, and then began to write to the data area of the real content of the file, such a file storage operation is completed. And when we delete the file, the steps are much simpler than creating the file. After we perform the delete operation, the system simply writes a deletion flag in the file allocation table before the file, indicating that the file has been deleted, that the space occupied by him has been "freed", and that other files can use the space he occupies. So, when we delete the file and want to get it back (data recovery), just use the tool to remove the flag, the data is restored. Of course, the prerequisite for recovery is that there is no new file write, and the space occupied by the file is not overwritten by the new content.