Asia Data Recovery Centre
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 Logical data recovery


Causation of logical data errors:

 Logical data errors are defined for the purpose of this article, as those errors that occur in dynamic or static media, which are either un-related, or only partially related to the physical condition of the media unit.   For this article, I will define the media unit, as a fixed drive, removable media [i.e. Zip disk, or floppy disk, tape, etc.], or as the dynamic memory embedded in a device, to which power has never been interrupted.

Logical errors can be generated either by error or malice.   Malicious programs such as public or private viruses can often be the source of serious logical errors.   The program may implement an agenda of data loss, or data manipulation, which is contrary to the agenda or objectives of the owners of the data or media unit.   A large number of public viral programs are well documented on a variety of anti-virus sites around the world, a couple of note includes Symantec, and McAfee, but there are also numerous academic and private sites, which detail the somewhat predictable actions of public viruses.   These programs are then classified along with private programs with malicious agendas.   Many such programs have be documented, they are distinguished from public virus by certain characteristics in that they are often highly dependent upon a specific system context, and depend upon privileged logical or physical access to the system, network, or media unit.

 In either case, public or private, the possible targets of malice are limited to either a system area, a resource of specific criteria, or a specific target.   A number of system areas are possible targets.   According to the OS, used to format the drive, [i.e. FAT16, FAT32, NTFS, HPFS, etc.], the system areas will include a master resource of the media unit. These master resources may include a bootstrap, allocation, or organizational resource.  If any of these resources is adulterated with either erroneous or malicious data, the media unit may be rendered partially, completely, contextually, or apparently dis-functional.   Examples of such damage are when a DOS based unit indicated that no valid boot resource is available, or a MAC fails to initialize.

 The adulteration of a key system area, will often effectively affect the media unit by the introduction of false program paths, or invalid resource tables.   If false program paths are introduced, be they well designed or simply erroneous, will cause the system processor(s) to execute commands which are either invalid or at the very least not in the interests of the user.   System level programmers are intimately familiar with such conditions, as execution paths which lead to data areas, or which cause infinite loops.   When these conditions occur, the results are highly unpredictable, and possibly injurious to existing data on the media unit.   The possibilities of injury result primarily from erroneous or malicious commands that cause good data areas to be overwritten, deleted, or damaged.   The other source of possible functional damage, which is slightly more common, and preferable to invalid execution paths, is that of adulterated system resources.  In these instances, the OS or application damages a key system resource in such a way as to cause the resource to fail initialization or integrity tests.  This type of damage may also, by containing invalid jump addresses, cause secondary processes to fail. If for example the partition table of a media unit is damaged, then it may appear to have partitions, which are inconsistent with the actual formatting of the drive.


Symptoms of logical data errors:

Media units with logical data errors may exhibit a number of characteristic problems, however the best POST tests are based upon elimination.   If the media unit is recognized by the system initialization procedure [BIOS], this effectively rules out many serious hardware errors.   Beyond this test the water gets murky, between hardware and logical errors.  

 Many physical hardware errors can cause data errors, which might also be the result of logical data errors.  Generally the saving grace is that logical errors are generally fairly systematic in their expression.   For example a disk, which is recognized, by the BIOS, but reports as having no boot partition is most likely the result of a logical data error.   While if that same disk was unable to boot, due to file errors, then the problem is most likely physical.  Specific common manifestations of logical data errors include missing files, folders, or partitions.   Also when a file contains no data, partial data, or incorrect data this is most likely a logical data error.


Recovery from logical data errors:

 Most logical data errors are recoverable.  If user data has not been overwritten by the actions of the logical error, it will most likely be recoverable.   The methods to affect the recovery vary widely according to the source and extent of the errors.

 In most cases a senior system programmer will be able to look at the drive with an editor, locate important data and move it into another file.  This generally works well if the files are small, or textual.   Larger, or non-textual files, like graphics or sounds, will require either intact organizational resources, such as FAT tables, or a machine language expert will attempt to reconstruct the file.   There are proprietary programs that are used in the data recovery field that may aid this process, but they can be extremely damaging in the hands of even experts who are not intimately familiar with the delicate data structures, which make up modern media units.


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