I had an interesting conversation with my brother regarding the lost data. He tried to explain in simple words. He said that the data deleted from the hard-disk was still there though it was not accessible through the normal route. There are special softwares using which deleted data can be retrieved. I asked my brother if the data has been deleted how can it be retrieved. And i found that such deleted files stay in the hard-disk just like floating kites whose string has been cut. They exist, but you cannot pull them back as the string has been lost. Interesting!
Another thing which he pointed out that there is one way by which the data will be lost forever- if "new stuff" is saved on the same disk area. So, he asked us to disconnect the computer from internet and not to save any new files on it. Hmmm....but what if somebody wants to erase the old files and save new files?
That's how a computer works. Human memory has only a few parallels with it. Fortunately or unfortunately one memory is not replaceable with another. Our memory has indefinite "disk-space". And it is not possible to delete specific memories and leave others.