Overwritten files

Disaster rating: 5/5

If your file has been truly deleted and then overwritten, the sad news is that you probably won't be able to retrieve it. However, there are two theoretical methods that seem promising, so perhaps the situation will be more hopeful in the future.

The first method is to do with variations in magnetic flux. When data is written to disk, the resultant magnetic flux depends mostly on the value written. However, the flux is also provided with a tiny contribution from the overwritten data.

So, if you replace the normal read electronics that decide whether a bit is a 1 or a 0 with circuitry that can extract the analogue value from the head, subtracting the known contribution of the most recently written data should make it possible to determine what value was in place before it.

This method of extracting overwritten data is commonly reported and academic papers have been written on the subject, but we've been unable to find any organisation that claims to have done it successfully. The problem is that the signal to the previously written data is so small that it effectively gets lost in the random electrical fluctuations commonly referred to as noise.

However, Western Digital's Gerardo Bertero – while questioning whether the technique is really a practical proposition – did suggest one possible solution. By reading each bit thousands if not millions of times and averaging all those signals, electrical noise, being random in nature, would average out to zero – whereas the genuine signal would build up and become visible.

The snag is that this would be hugely time-consuming and costly – which is why nobody offers such a service. Whether it becomes feasible when national security is at risk is another question entirely, and one we're not likely to get an answer to from the Secret Service.

The second theoretical technique for recovering data is to use a Magnetic Force Microscope (MFM). It's claimed that this method offers an additional benefit – it can read data, overwritten or not, from the surface of a platter that is no longer able to spin because of damage.

For the normal read operation of a hard disk, the platter must be able to rotate because an electrical signal can only be produced in the read head's coil if it's moving with respect to the magnetic field. However, an MFM is able to read a static magnetic field – so the platter doesn't have to be able to spin.

An MFM is a laboratory instrument that has an ultra-fine magnetised tip suspended on a cantilever. As the tip moves over the surface of the object being imaged, the magnetic field of that object exerts a force on the tip, which in turn causes a displacement of the cantilever. This is measured using optical techniques.

It's self-evident how an MFM could be used to read data from a platter that can no longer rotate, but what isn't as obvious is how the technique lends itself to recovering data that's been overwritten – at least in theory.

Data is written to a hard disk's platter in concentric circles known as tracks. A highly accurate servo control system is used to position the read/write head over the required track, but, even so, the head isn't always positioned to exactly the same radial position. What this means is that if a track is overwritten but the head wasn't at exactly the same position as it was on the previous occasion, a narrow band of the previously written data might remain intact at the edge of the track.

Again, despite hearing so much about this technique, and despite the fact that hard disk drive manufacturers already use MFMs for research and development, we haven't found any company who offers this technique for recovering overwritten data. We'd have to speculate on whether it's in use by the military, although the fact that military standards require disks to be physically destroyed at the end of their lives might just suggest that they know it's feasible.

Scratched discs

Disaster rating: 1/5

Methods such as undeleting files, repairing logical errors to the file structure and reassembling files also work with media such as memory cards and pen drives. However, there are certain methods of data recovery that apply only to optical disks. Except in the most extreme cases, when a CD or DVD won't read it's usually scratches in the outer plastic layer that are causing the problem.

The plastic layer is there to provide protection to the layer of data underneath, but scratches can impair the passage of the light used to read the data. The solution is to remove the troublesome scratches using a mild abrasive so that light will pass through the plastic layer without obstruction.

Although there are reports of this being done successfully using household substances such as Brasso, a more reliable solution is to use a product designed for this job. Such products range from kits comprising a bottle of suitably mild abrasive and a lint-free cloth to mechanical disk polishing devices such as Digital Innovations' SkipDr. The latter claims a greater chance of success since the polishing process is more uniform and controlled.

Shattered discs
Disaster rating: 4/5

Researchers at the University of Arizona's Optical Sciences Center have published a peer-reviewed paper in which they describe how they used an optical microscope to extract data from fragments of broken CDs or DVDs.

However, the process is time-consuming and so isn't likely to be viable unless the rewards are huge. What's more, any data in the vicinity of the breaks is totally unrecoverable, so in many cases we'd be talking of recovering fragments of data rather than files in their entirety. A shattered disc still means lost data – although the future may hold more hope for broken optical discs.

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First published in PC Plus Issue 291

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