For simple cases, Encase is forensics software that can be used for viewing deleted data. When you delete a file, it is not really gone from the drive, rather it is simply not indexed anymore and the space it is taking up can be overwritten by other files. However, it can take a long time before anything is written over the file, so even many years later a deleted file can be recovered with Encase. In more difficult cases, someone might try to physically destroy their platter by smashing it into a bunch of pieces or otherwise doing physical damage to it. In these cases they reconstruct the platter from its pieces and use spin stand microscopy, which essentially uses a magnetic microscope to read from the platter pieces, which are placed on a spinning stand to mimic the platter rotation. Even more advanced recovery may attempt to gather data fragments from the edge of a platters track , that is why Secure Erase puts the magnetic head slightly off track during its second pass. It may be possible to recover some data even if the drive is written over, unless magnetic residue on the track edge is taken care of. I read a study about data recovery from edge residue once, but I cannot remember the conclusion it came to. I imagine it is a real concern though, as Secure Erase attempts to counter it. For extremely advanced stuff, In the past there was a theoretical paper written about using magnetic force variance measurements to try to determine the history of a given bit on the platter, in order to recover overwritten data. The infamous 35 pass guttman wipe was designed to overcome this. On modern drives this is widely thought to be a non-issue due to greatly increased platter density, also I am not certain if this technique has ever actually been used in practice or if it has been confined entirely to the realm of theory. Even in the forensic data recovery world there is a bit of debate regarding this technique, some people seem to think it is valid but I have also seen others saying it simply does not work and is a myth. I am not worried about it anyway .