How do computers permanently store data? [duplicate]

Possible Duplicate:
Where do deleted items go on the hard drive?

I've heard several times that when you delete a file, even after removing it from the Recycle Bin, it is still permanently stored on your computer and can be accessed by the likes of police professionals if needed. How does this work without effecting disk space?

This data isnt stored strictly speaking.

What happens when you delete isnt EXACTLY the computer going and wiping all the data out. What actually happens is that the computer forgets that that file exists(delete file system entry for that file). This means that if another file comes in that needs harddisk space, it might be stored over the old file. If this does not happen, several tools can scan the hard disks to undelete such files.

You can try this yourself by googling undelete. You will get many tools which will be able to recover deleted files.

This is why, when trying to securely delete files, people use specialized tools which actually go over every bit of hard disk space used to store those files and fill it with all 0s or random junk.

You can find more info at http://www.howtogeek.com/72130/learn-how-to-securely-delete-files-in-windows/

A good way to think of it is as a library. We have a set of index cards (the file table) relating to the books (files) on a shelves (clusters). If you need a file, you look up the index, go to the right place and get it. When a file is 'deleted' the index of the cluster is marked as 'deleted' but the actual file is still there. When the 'shelf' is full, the system looks at the file table, finds a location thats marked as deleted and overwrites it. This saves on writes. The actual file, or parts of it is not overwritten till the space is needed - which makes it recoverable.

The computer ignores 'deleted' files when calculating space since, well, as far as its concerned, there's nothing important there.

A 'secure' deletion is simply overwriting the area with zeros or random information.