The Science of Data Recovery

Chris Gerhard made an off hand comment about the fact that disk scrubbing simply hinders (doesn't necessarily prevent) a motivated attempt to retrieve information from a disk drive. Disk Scrubbing is the process of (attempting to) securely erasing a disk to prevent others from accessing previously stored information. This is typically done by writing (possibly multiple times) random data over the entire surface of a disk.

Since I work with various government accounts/agencies/programs, this is an area of interest to me and some of my clients.

You might think that a digital medium designed to store only zeros and ones would be immune to forensic recovery of residual data once the zeros and ones are randomly altered. The fallacy with this is that magnetic storage is not a digital medium at all. Magnetic domains are created when the read/write head applies energy to a bit location to align some (not all) of the particles to reflect either a zero or a one. The precise location of the "domain" for each write varys slightly in three dimensions (including depth). This reality provides interesting opportunities or risk (depending on your perspective).

A colleague (thanks Joe) pointed me to a fascinating report on techniques involved in recovering data from ostensibly erased disks and computer memory. This is amazing and spooky stuff for the technically inclined. Here is another report (thanks Kurt) that's also very interesting and enlightening. Joe also pointed me to Prof. Gutman's website, who has a lifetime of security related knowledge to share!

Here are a few brief excerpts (read the article for context):

When all the above factors are combined it turns out that each (disk) track contains an image of everything ever written to it, but that the contribution from each "layer" gets progressively smaller the further back it was made. Intelligence organisations have a lot of expertise in recovering these palimpsestuous images.

To effectively erase a medium to the extent that recovery of data from it becomes uneconomical requires a magnetic force of about five times the coercivity of the medium... (a modern hard drive has a  coercivity of 1400-2200 Oe).... Even the most powerful commercial AC degausser cannot generate Oe needed for full erasure. It may be necessary to resort to physical destruction of the media to completely sanitise it (in fact since degaussing destroys the sync bytes, ID fields, error correction information, and other paraphernalia needed to identify sectors on the media, thus rendering the drive unusable, it makes the degaussing process mostly equivalent to physical destruction).

One example of an adequate degausser was the 2.5 MW Navy research magnet used by a former Pentagon site manager to degauss a 14" hard drive. It bent the platters on the drive...


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Posted by erdod on January 09, 2006 at 02:25 PM EST #

Good article.

Posted by aqedol on January 18, 2006 at 04:17 PM EST #

I've spent the past 5 weeks building a new life on my new hard drive. I've been working here for a year and a half and no, I didn't have any backup of any kind. It's like being a new employee again. No e-mail. No bookmarks. I've already heard it all from my loving and caring colleagues here at SNC, so don't start...

Posted by 数据恢复 on April 05, 2006 at 06:32 PM EDT #

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Posted by jim on April 17, 2006 at 03:32 PM EDT #

The data recovery software are a great help. But for many people who dont have time to try all these just like there are many organisations helping in the data recovery. one such being from where i got my recovery, liked the way these guys worked and on time delivery.

Posted by Robin on June 14, 2006 at 01:27 AM EDT #

Wow, I don't think that I will ever have to go that far to get the data I need. I'm just happy that my Stompsoft's Recover Lost Data can get back the files I need. Though, it is simultaneously scary and reassuring that there are ways to retrieve nearly anything for the right price.

Posted by Will on September 08, 2006 at 07:41 PM EDT #

I hear you Will. I found it to be very disheartening to know that my whole computer life could be pried open by a determined individual. Though, on one hand I understand that such measures are needed to catch the "bad guys" but on the flip side criminals could reek havoc with your personal information. I guess though if I stop and think for a moment, I realize that make use of the same software that you do and subsequently I guess we all enjoy the ability to get back what, by fate, we should not have.

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Posted by anyu on August 16, 2007 at 07:40 PM EDT #

Hey Dave

Nice writeup about the data recovery science and the microscopic issues..

Good Work!!!!

Posted by Computer Data Recovery Software on August 17, 2007 at 11:03 PM EDT #

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Posted by raj on August 19, 2007 at 10:41 PM EDT #

how to recover my data

Posted by raj on August 19, 2007 at 10:48 PM EDT #

@ raj,

I dont think that any of the data recovery software can corrupt or cause damage to your data. It only happens when users try to recover data on the bad device which then corrupts the original data by replacing it with.

The rule goes that nothing should be tried on damaged drive as it will overwrite the original contents, alwayas perfrom data recovery on spare drive or any external media, then it will not harm the original data.

Could not understand your views of first going against the data recovery software in your 1st post and then asking how to recover data in 2nd post????

Posted by computer recovery on August 19, 2007 at 10:57 PM EDT #

Each data recovery software has limitations, for example it will help if you have a logical problem, but it will not work if you have a noisy drive or your drive has a mechanical failure.

Posted by Promise Data Recovery Chicago on August 23, 2007 at 05:42 AM EDT #

yes, some of the data recovery software might have limitations for noisy or mechanical failures but prevention is the best cure. Whenever such sounds are heard from the drive or you come to know any hint of mechanical failures; one should start backing up data immidiately.

If you never get the chance, then data recovery services are the best solution. Data recovery technicians can recover your data by working on the damaged disk...

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Posted by sunita on August 29, 2007 at 11:45 PM EDT #

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Posted by data recovery on September 04, 2007 at 03:56 PM EDT #

If you find that your hard drive is clicking, the first and foremost thing you must do is shut down your system to prevent further damage. This will reduce the chances of permanent damage and will also reduce the risk of making your data unrecoverable. If the hard drive is clicking, making grinding/scraping or whirring sounds, the BIOS does not recognize and accept the drive; external hard drive, laptop or system or was jarred or dropped; the computer was exposed to water or fire/smoke damage. When a hard drive is exposed to any of these things, it will result in physical and internal damage to the read/write head of the hard drive and/or the platters.
When the hard drive is in action, the read/write heads do not rest on the platters. During operating, the heads ride on a micro-cushion of air that prevents it from making any physical contact with the surface of the platters. When the head of the hard drive begins scratching the platters, it results in loss of information. This is known as clicking. If you try to keeping switching off and booting your computer again and again, you are causing more damage and this can result in permanent data loss. A clicking hard drive is a very serious symptom of hard drive crash and must be taken seriously.
The best thing to do when your hard drive crashes is to switch off the system and not boot it up again. Keep it switched off and let a professional technician deal with it. Sometimes disassembly of the hard drive is required in order to recover data. It depends on the extent of the damage to the hard drive. In clean room environment, the covers and seals are removed from the hard drive so that technicians can work on or replace internal parts while maintaining the warranties.
The technician first begins by identifying the problem and replacing problem causing internal parts like read/write heads, re-writing firmware, performing platter swaps, recalibrating and realigning file structures and restoring as much of the data as possible. Hard drives that click generally go a clean environment room where they are disassembled and the individual components are checked for scratching, scoring, correct calibration, ohm reading and failed chips. ROM chips and firmware is also checking. The spindle motor will be tested for proper rotations, speed, alignment and frozen or damaged bearings. Mechanisms that can identify the damaged are used to test the hard drive for overwriting and damage. Faulty parts are replaced and new firmware is installed. When the hard drive is brought to a ready state, the raw image is created sector by sector. After the imaging process, data is recovered, critical files are checked for damage and integrity and the data is saved to a new physical location like an external hard drive.
If the hard drive is not suffering from any severe mechanical or electrical failure then it is sent to a cloning room where bit by bit and sector by sector raw images are created. Once an image of the hard drive is obtained, it is read to make sure the files are not corrupt and the critical files and folders are recovered and copied onto a new physical location. Imaging can take anywhere between 2 to 16 hours depending on the extremity of data loss and the size of the hard drive.
Hard drive data recovery depends on the severity of damage and failure. It is best to keep regular backups of important files and folders to avoid complications due to a hard drive crash. Always make sure you handle an external hard drive carefully and do not hit your computer if it hangs – the hard drive just might crash due to the force!

Posted by RAID data recovery on September 22, 2007 at 02:12 PM EDT #

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