Data Recovery Techniques
A wide variety of faults or failures can cause physical damage to storage media. CD-ROMs can have their metallic substrate layer scratched off; hard disks can suffer any of many mechanical failures, such as head crashes and failed motors; tapes can simply break and other forms of media could have a dry joint. Physical damage always causes at least some data loss, and in many cases the logical structures of the file system are damaged as well. Any logical damage must be dealt with before files can be salvaged from the failed media.

Most physical damage cannot be repaired by end users. For example, opening a hard disk in a normal environment can allow airborne dust to settle on the platter and become caught between the platter and the read/write head, causing new head crashes that further damage the platter and thus compromise the recovery process. Furthermore, end users generally do not have the hardware or technical expertise required to make these repairs. Consequently, data recovery companies are deployed to salvage important data.

Recovering data from physically damaged hardware can involve multiple techniques. Some damage can be repaired by replacing parts in the hard disk. This alone may make the disk usable, but there may still be logical damage. A specialized disk-imaging procedure is used to recover every readable bit from the surface. Once this image is acquired and saved on a reliable medium, the image can be safely analyzed for logical damage and will possibly allow for much of the original file system to be reconstructed.
Physical Crash: When your hard drive is making a ticking or a scratching noise due to age or bad parts, the aperture arm in the hard drive can fail, failed motors or the platters can become damaged and lose the data that they hold. Physical damage could also be due to mishandling of the hard drive or circuit failure of the PCB board which is attached to the hard drive. Physical damage always causes at least some data loss, and in many cases due to physical damage the logical structures of the file system also may get damaged, all such damages constitute physical crashes of hard drive.
Logical Crash: Far more common than physical damage is logical damage to a file system. Logical damage is primarily caused by human error which includes accidental deletion of files, accidentally formatting drive and power outages may cause file system structures getting damaged. Causing a variety of problems, system crashes, or an actual loss of data.

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