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Wipe Out Old Data

by Alan German

The club’s monthly meeting in June will feature our annual E-waste event, providing all members with the opportunity to dispose of any unwanted electronic equipment in an environmentally-friendly manner. Perhaps you, a friend, or a relative have an old computer that is no longer useful and you would like to send it for recycling. However, maybe you are hesitating because the hard drive contains files with personal information.

The recycler will physically shred the hardware, including the hard drive, which will render the data irrecoverable. But, perhaps you have lingering doubts that someone will be able to access your sensitive files before the disk is destroyed. If so, you need to securely erase the files or the entire hard drive before you hand over the computer for recycling. This can readily be achieved through the use of appropriate software. But, before we delve into some suitable programs, let’s take a simplified look at how files are stored on a hard disk, and how we can make sure that they are completely erased.

A conventional hard disk has one or more magnetic platters on which data are stored. The disk manufacturer performs a low-level format which splits the storage area into many circular bands or tracks, each of which is further broken up into individual disk sectors. Depending on its size, a specific file may be stored in a single disk sector, or may occupy several disk sectors. The Windows operating system keeps track of the location(s) of each file using file pointers. Older versions of Windows typically use the FAT file system in which the file information is stored in a File Allocation Table. FAT systems are still in use, primarily for devices such as USB memory sticks. More recent versions of Windows use the New Technology File System (NTFS) and store file information in a Master File Table.

When you delete a file in Windows, it isn't actually erased. First, the file is moved to the Recycle Bin and the operating system updates the file pointers to reflect the file's changed location. If you delete the file from the Recycle Bin, it still isn't erased. The operating system merely marks the space allocated to the file as being available for future use in storing other files. The Recycle Bin acts as a sort of fail-safe mechanism whereby you can easily restore a “deleted” file in a situation where the deletion was unintended. But, even if the file is removed from the Recycle Bin, because it is only “marked” for deletion, the file's data are still present on the hard drive, and special recovery software can still restore the file, providing that the data have not yet been overwritten.

So, there's the source of concern. You can delete your sensitive files, but you have no guarantee that the files cannot be recovered. Clearly you need to make sure that the actual contents of the deleted files have been destroyed. You need to “securely erase” such deleted files, or perhaps even erase – or wipe – the entire contents of the hard drive to make sure that none of your personal information can be read.

The trick for completely erasing a file is to overwrite the “free” space that is still occupied by the data for a deleted file with other information. For example, all of the bits occupied by the file's data could be overwritten with zeros. However, some sophisticated techniques can still restore so-called “weakly-deleted” files by reading patterns that remain in the magnetic media even after a file has nominally been overwritten. Most file-shredding and disk-wiping software offer multiple methods for overwriting a disk's free space. These generally consist of multiple passes to overwrite the free space, often using random data, in order to minimize or even eliminate any possibility of data recovery.

Obviously, the more passes that are made, the longer it takes to wipe the free space. The option selected will depend on the size of the free space, or of the disk being wiped, and the degree of security that you as the end user wish to implement. However, bear in mind that data recovery on a disk that has been wiped by even a rudimentary algorithm is a non-trivial task. No doubt government spooks will be able to recover the data, but it's unlikely that mere mortals will be able to do so. Also, you need to consider what's the possibility of anyone trying to recover any information before the hard disk hits the shredder.

The good news is that you don't need to do very much to securely erase deleted files or completely wipe an entire drive. There is lots of software available that will accomplish these tasks. A Google search for “secure erase” or “wipe disk” will provide links to dozens of candidate programs. My preference is always for free software, and there are several such offerings for file shredding/disk-wiping programs.

There are two considerations for these operations. If you simply want to securely erase the free space on a hard disk, which will include files that are marked for deletion, you can install file-shredding software onto the hard drive and run the program from Windows. However, if you want to wipe an entire drive that is running Windows, you can't do this from the Windows drive itself. Clearly, you can't use a Windows utility program to delete all the files on the disk running the operating system. This would require deleting both the disk-wiping program and Windows while both are running! To accomplish this task, you need to use a stand-alone, bootable disk that contains the disk-wiping software.

Firstly, let's consider how to shred the free space on an operating disk drive. I have previously indicated one freeware program that will perform this task ( File Shredder is currently at Version 2.5 and is released under the GNU/GPL General Public License. Once installed, File Shredder allows you to specify one or more files or folders to be shredded. Alternatively, you can shred the free disk space across an entire disk volume. Another option is to remove all the files from a disk drive. The latter operation is most useful for wiping external drives, such as a USB memory stick, or a standard hard drive installed in an external USB disk enclosure. You are given a choice of shredding algorithm. File Shredder offers five different algorithms, commencing with a single pass, up to the use of the Guttman algorithm using 35 passes – which is probably overkill for most of us!



If we want to securely erase an entire disk drive we have a couple of options. We could remove the hard drive from the old computer and plug it into a disk enclosure. The disk enclosure is then connected to a second Windows computer through its USB interface. Software, such as File Shredder, can then be used to remove all the files and folders on the external drive. However, it may well be easier to erase the hard drive in place in the old computer. This requires using specialist software installed on a bootable CD/DVD or USB drive. Obviously, the choice of bootable media will depend on what disk formats are supported by the target computer’s hardware.

One freeware option that can be used for this purpose is HDShredder. The download includes an ISO file which can burnt to a CD, DVD, or a USB drive. The resulting disk is dedicated to the shredding process such that, when booted, the computer runs the HDShredder program directly. A user-friendly graphical user interface provides a series of screens guiding the user through the steps required to specify which disk is to be wiped and which method is to be used.

The initial screen allows the user to select if a disk or a disk partition is to be erased. Since we are erasing the entire drive, we simply retain the default option to erase a disk. The second screen list the drives that are connected to the host computer. Using the mouse, or the up and down arrow keys, the specific disk to be erased can be selected. The next screen offers a choice of methods, nominally including automatic, standards, and user-defined. The automatic process is the only one supported by the free version of HDShredder and is selected by default. Further options are to use fast (1 pass), medium (3 passes), or high security (7 passes). The subsequent options screen activates or disables items such as cached and shared memory which the help file suggests should be left enabled unless any problems occur.

The final screen provides a control button to start the disk wiping operation. Pressing this button brings up a dialogue box warning that the contents of the selected disk will be completely erased. Pressing the Start Deleting button brings up a progress bar and some statistics, including the percentage complete and the run time. At the end of the process a pop-up window provides a final report indicating the drive’s name, the method and number of passes used, the number of deleted sectors, any write errors, and the total run time.



The free version of HDShredder is limited to the use of the so-called “write zero” method of erasure. A zero value is written to each bit of data on the drive, thus over-writing any data that was previously present. As noted, for additional security, the over-writing process can be repeated multiple times, using either 3 or 7 passes.

The relatively simple method adopted by the free version of HDShredder should be acceptable for most users. Anyone wishing to use HDShredder, but requiring additional security, would need to use one of the commercial versions of the software that activate up to 15 additional algorithms for disk erasure, plus the option for a user-defined protocol. An alternative would be to use a different freeware program such as Darik's Boot and Nuke (DBAN). The user interface for DBAN is somewhat less sleek than that of HDShredder, but the disk wiping algorithms are much more powerful.

Clearly. more complex erasure techniques, especially those involving multiple passes, require additional run time. Even a single pass on a relatively large hard drive will take considerable time. The disk wiping process is not going to be a five-minute job. The reason that files are typically marked for deletion rather than actually erased is that changing a few bytes in the file system to note that the space previously occupied by the files is available for reuse is much faster than zeroing every byte of data occupied by the files. Erasing an entire disk can mean writing zeros to literally billions of bytes. Even at the data transfer rates use by computers this process is still time consuming. Secure disk erasure is therefore best achieved by using a set-it and forget-it method. Set up the process to blank the disk, start the software, and then leave the computer to do its magic.

My final words of wisdom on the topic of file shredding – and especially for disk wiping – are to carefully check the files, folders, or disk that you have marked to be wiped, before hitting the Start button. The whole purpose of the software is to completely destroy the contents of the items selected. Obviously, if such programs do their job (and they do!) you won’t be able to recover any data that you mistakenly flagged for erasure.

You can use the techniques described here before you sell or recycle an old computer. The good news is that any recipient of the machine with the wiped hard drive won’t be able to recover any data. At least, that will be the case unless the recipient is a government spook. So, just don’t sell your old computer to a government spook!

Bottom Line:

File Shredder (Freeware)
Version 2.5

HDShredder (Freeware)
Version 4
Miray Software>

Originally published: May, 2016

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