Computer data is typically stored in magnetic form on Hard Disks, Magnetic Tapes, Compact Discs, DVD and RAID. Data on these storage media could become inaccessible owing to physical damage or corruption of the logical file system.

The logical file system is a way of organizing and accessing the file system and its contents. For example, the disk surface might be divided up into sectors of, say, 512 KB, and each sector allocated a distinctive ID.

The file system would keep track of the sectors used by each file. These sectors would usually not be contiguous because delete operations might free up some sectors that might not be sufficient to complete the next write operation. In such cases, writing would be done on sectors that are physically separate. It follows that the file system has also to keep track of which sectors are free for the next write operation.

The file system logic is a complex one designed to meet the requirements of many possible read/write/delete scenarios in an optimum manner. If it gets corrupted (owing to, say, power problems or system crashes) the contents of the disk would become indecipherable.

We saw above that data could become indecipherable when the file system gets corrupted. In addition to file system damage, physical damage to storage media could also lead to loss of data. Physical damage could usually cause a more serious data loss. We will look at Hard Disk Drive damage to get a picture of what could happen.

The main components of the Hard Disk Drive are the platters on which data is stored, and the read/write heads above each platter. The read/write heads sense or record magnetic information on the platters. In normal operation, the platters rotate at extremely high speeds and there would be a microscopic film of air between the platter surface and the read/write head.

If for some reason, the read/write head comes into physical contact with fast rotating platter, the magnetic media on the platter surface gets damaged. This event is known as a "head crash".

Another possibility is for the motor that rotates the platters to become faulty. The highly precise read/write operations would then either not be completed successfully, or fail to yield correct results. There are other mechanical parts in the drive, any of which could fail leading to similar results.

In cases of real physical damage such as those caused by fires and floods, the data could also be "really" lost. In these cases, unless you have a reliable backup of the data stored in an unaffected location, the only option would be to reconstruct the whole data from original documents or other sources.

In most other cases, however, it would be possible to recover the "lost" data if you entrust the job immediately to expert data recovery agencies with required facilities. They might then be able to extract the raw image on the disk platters and use it to reconstruct usable data.

The recovery would typically proceed by repairing any damage to the file system, and through physical means such as replacing a damaged read/write head with a matching, healthy head, moving the disk platters to a matching, healthy disk drive and/or replacing the drive's damaged PCB with a matching healthy one.

All the operations would be done in specialized "clean rooms" because even microscopic particles of dust could damage the image on the disk platters. Specialist data recovery agencies would also typically be "authorized" by drive manufacturers to open the drives without voiding the warranty.

Data is packed more and more densely with each new generation of drives, and the reconstruction could require extremely precise operations, as well as facilities like Class 100 clean rooms.

If you try to recover the data yourself by opening the drive, you could make the data permanently irrecoverable, not to mention voiding the manufacturer's warranty on the drive. The minute specks of dust present even in air-conditioned rooms could settle on the platters damaging the images on these.

Even the use of software tools for data recovery, offered by utility software developers, could not only fail to recover the data but also make it more difficult (and expensive) to recover, if at all.

Additionally, steps like replacing read/write heads and PCBs, and moving disk platters to another drive, are best done by trained personnel. They would have gained experience of the different eventualities that could occur, and know how to take necessary precautions.

Call in the experts.