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Deleting files permanently

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Most people think that when something is not visible it does not exist anymore, but this is far from truth, especially when we are talking about computers, which only show a tiny bit of what really happens inside them.

In the case of deleted files in the hard disk, the computer (or actually the operating system) lies. When a file is sent to the recycle bin, despite Windows says it has been permanently erased, it actually is not deleted but just tagged as overwritable.

Because of this fact, the restoration of any file (even after being sent to the recyble bin) is possible. However, this is a fact that also leads to privacy problems. In order to delete permanently a file from your hard disk you need special applications. In this article, I'll explain you which applications you need and how they work.

But before continuing you must understand the structure of hard disks and how files are stored in them.

How files are stored on a hard disk

Inside the hard disk, there is a platter divided in tracks, which are concentric circles. Each track is divided in pie-shaped sectors as shown in the image below.

Hard disk tracks and sectors
The tracks are the concentric circles like the one shown in yellow, while the sectors are the pie-shaped divisions of the tracks, like the one shown in light blue.

Sectors have a fixed size with a fixed number of bytes, so each sector can have a size of 512, 1024, 2048 or 4096 bytes, depending if the hard disk is old or new (the size of sectors in newer hard disk drives is of 4096 bytes). If you multiply the number of sectors per track by the total number of tracks by the size in bytes of each sector you get the total size in bytes of the hard disk.


In Windows, sectors are grouped into clusters that vary in size and number of sectors. In large disks the default size of clusters is 4 KB (4 kilobytes or 4096 bytes). So in newer hard disk drives a sector size matches a Windows cluster size.

Files are stored in groups of sectors and each sector is numbered. So if we have a 1 MB file (1 megabyte or 1,048,576 bytes) and each sector has a fixed size of 4096 bytes (4 KB), that file will be allocated 256 sectors, for instance, from sector number 400,244 to sector number 400,500.

For files smaller than a sector size, a whole sector is allocated. For instance, if we have a 4000 bytes file and the size of each sector is 4096 bytes, that file will be allocated 4096 bytes, taking up the whole sector.

If we have a 20,000 bytes file it will be allocated 5 sectors (20480 bytes), even when there is a spare space of 480 bytes in the fifth sector, it will take up the whole 5 sectors or 20480 bytes.

Deleting files process

When a file is deleted and sent to the recycle bin, it is not actually deleted but the sectors it has been allocated are tagged as overwritable. It means, those sectors can be overwritten at any time by other data if the operating system considers that space as convenient for a new file. But until that happens, the data of the file we "deleted" remains untouched.

Now you may wonder why the operating system doesn't delete it permanently instead of just tagging it as overwritable and leaving it untouched. The reason is simple, overwriting fully the sectors of a deleted file with new data (perhaps filling it with zeros 0s) each time a file is erased, would be a costly and slow process that would take a lot of time, decreasing the overall performance of the disk as well as of the operating system.

For instance, if we delete the file of one of the previous examples, the 1 MB one, that has been allocated 256 sectors, from the number 400,244 to the number 400,500; those 256 sectors will be tagged as overwritable; letting the operating system to allocate those sectors to a new file at any time.

In practice, this system is useful since most users don't require a full data deletion for security reasons, thus making the deletion process really fast. But in some cases, a permanent and irreversible data deletion might be necessary.

For example, governmental organizations and institutions or big companies that handle sensitive data, might need the permanent destruction of that information for different reasons. In the case of common users the permanent destruction of data might be a necessity related to privacy preservation.

In such cases some special tools and applications are required in order to achieve the permanent deletion of sensitive information.

How to permanently delete data from the hard disk

As mentioned before, deleting a file and sending it to the recycle bin is not enough to achieve its permanent destruction. Even partitioning and formatting the hard disk drive is not enough; as there are forensic information technology applications and tools that if used correctly can easily restore all the deleted information.

That's why in order to remove permanently sensitive data, companies, agencies and institutions might require the use of different methods like the Gutmann algorithm and the DoD 5220 method. These permanent data destruction methods differ in efectiveness and performance.

Both methods perform complex tasks in order to remove all the former data from a hard disk drive, a process that usually requires a long time to be finished. For example, the Gutmann method can perform 35 passes, overwriting all the hard disk over and over.

But for most average home users the complex methods described previously are not necessary, so for them there is a series of different applications that can perform secure permanent data deletion in their hard disks. For example, these applications are very useful in cases when the user wants to sell a computer preventing identity theft or when the user needs to totally clean the operating system of strong recurrent viruses and spyware programs. Here is a list with some of the best tools available:

Windows permanent data deletion tools

  • Eraser: This is one of the best permanent data destruction applications available. One of its pros is that it is easily integrated with the operation system, while some of its cons are that it can become unstable in certain systems and that it doesn't provide a help. You can download it from here: eraser.heidi.ie/download
  • Hardwipe: Another very effective tool to destroy permanently single or multiple files, wipe hard disk drives entirely, as well as cleaning free space previously occupied by older data. You can download it from here: www.hardwipe.com/download.html

  • Freeraser: This is a good effective alternative to the other aforementioned tools. You can download it from here: www.freeraser.com

In more drastic cases there is a very powerful tool for Windows that can wipe your hard disk drive completely. It is very useful for emergencies. This tool is DBAN (Darik's Boot And Nuke). You can download it from here: www.dban.org

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