It is more difficult than you would think to irrevocably delete data, as deleting and deleting can be two completely different things! Only deletion by repeated overwriting will irrevocably erase a file or a whole data carrier for good. Another option is to physically destroy the data carrier as long as it is absolutely certain you will not want to use it again.
- using tools such as Eraser, Secure Eraser or DBAN
to repeatedly overwrite files with various character strings
- physically destroying the data carrier
Just deleting from your recycle bin is not really enough!
When deleting files from your computer, they are usually moved to the recycle bin first. If needs be, you can still retrieve those files from there. It seems that files are only erased for good once you empty your recycle bin. Yet that's just not the case.
With this normal type of deletion, areas containing files to be erased from your hard disk are not wiped, but are simply marked as available for overwriting. All reference to your erased files is deleted from the hard disk index, and the respective area is allocated for reuse. However, until another file is actually written over these marked areas, all files actually remain on the disk.
Even if the whole hard drive is to be wiped, standard formatting is often insufficient. Quick Format will only delete the index of your hard drive. Your files are still on your hard drive though, and can be restored using special tools. From Windows Vista onwards, data will also get overwritten when using the standard formatting option, so they are safely deleted and can no longer be restored.
It is therefore possible to retrieve files which have not yet been overwritten – something which can be very helpful if you have mistakenly deleted some files you still need. For security reasons how-ever, this is not an ideal solution if you would like to erase, say, a confidential file for good.
To securely delete a file or erase a complete hard drive requires specific software. Such programs overwrite those hard drive areas where files used to be – and if required, this process can be repeated several times, irrevocably deleting data.
To ensure no confidential documents can be retrieved, it is advisable to use such software to securely erase them. There are a number of both commercial and free programs on the market, such as «Eraser» or «Secure Eraser».
- Eraser: «Chip online» offers a special workshop on using this software (german)
Software download: eraser.heidi.ie
- Secure Eraser: an informative set of instructions can be found on the «Computerbild» magazine Internet site (german)
Software download: www.secure-eraser.com
Please note that a file can only be erased safely from an USB stick (flash memory) if you securely erase the whole stick. Individual files cannot generally be safely erased from an USB stick.
If you are selling or disposing of your old machine, you should delete all data from your hard drive first, as you do want to make absolutely sure that the buyer of your PC won't be able to view any of your old files, such as pictures or letters. The simplest way of wiping a whole hard drive is via a bootable CD, for instance free software such as DBAN.
Another method of destroying data is to physically demolish the data carrier, for instance by smashing a USB stick up with a hammer.