How can I clean up my PC’s hard drive myself, without damaging or deleting crucial system data?

Answer

Data can indeed be retrieved from a hard disk.

There is the question of what exactly is meant by deleting a file. A first interpretation of deletion is that the file is no longer available to the user in the operating system (Windows, Mac OS-X or Linux), and that the location of the deleted file *may* be used by a new file. This is what happens in almost all systems because it works fast. A second interpretation of deletion is that the file is literally deleted: where the file was, it is ensured that it is completely gone. This is slower than the first method and is therefore rarely used. Places where it is used is for example by the government, or for sensitive data.

You can fully compare both forms with files on paper. The first interpretation is when you throw your paper away in the garbage can. It’s gone for you, and you can use the space on your desk for a new paper. However, someone can come and search your trash can and find out things that way. The second interpretation is when you put your files in the shredder.

A second part of the puzzle is related to how a file is stored on a hard drive. Without going into too much detail, we can say that 1 file is stored as a lot of small parts on the hard disk. All the parts together, in order, are then your file. A table is also maintained that indicates for each file which parts are used, as well as which parts of the hard disk are currently not used (are empty). If we delete a file in the first interpretation then simply all parts of the deleted file are added to the empty parts in the table. In the second interpretation, a special program will first go through and empty all parts of the file, and only then will they be released.

There are programs that allow you to delete data in a safe way. On this page you will find a list of programs (the page is in English – http://www.thefreecountry.com/security/securedelete.shtml). If you throw away your computer, you can do something else: a so-called ‘low-level formatting’ of your hard disk. You need some time and knowledge for this, but then it becomes much more difficult to find data.

Analogously, there are also programs that allow to find data on a hard disk. They are going to search the hard drive in a very systematic way, looking at all the parts of your hard drive, one by one. Then they try to piece those particles back together into complete files. This works well as long as all parts of a file are still available and have not been used in newer files.