Why might it be impossible to recover lost data?

Answer 1

Generally when a file is deleted, the file system makes no effort to erase the actual data from where it is physically stored on the disk. The file system deletes files by simply breaking the file name.

In windows and DOS, the delete command simply replaces the first letter of the filename with a question mark. This indicates to the file system that the file is no longer needed and any space it occupies can now be treated as blank space, even though it isn't.

To recover the lost file, assuming no new files have been written to that space, all one needs to do is restore the file name. This can be done manually, but it is far simpler (and generally more effective) to use a file recovery utility.

If it is desired to permanently delete the data such that it cannot be recovered, the data must be over-written, preferably many times over.