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Damage to a drive's circuit board or defects of the drive heads occur fairly often. In both cases, to access hard drive data, a data recovery specialist will substitute the defective component with a working one. In the best case scenario you regain 100% of your data.
A so called head crash - where a write/read head physically hits the magnetic platter - is much more severe. Simply hitting your drive might cause contact, since the distance between the head and surface of drives is minimal nowadays. It is almost always safe to say that a damaged magnetized surface causes the loss of saved bits. The head might even repeatedly get caught at the area of defect, carrying off more material with each additional contact. This material will be distributed inside the drive, causing scratches or other damage. There is nothing a customer can do in case of an intense head crash, because the head is simply unable to move across the defective area any more.
Overvoltage on write/read heads causing the permanent destruction of data areas or magnetization will also result in irreversible data loss. This means the physical destruction of memory sectors, not the simple deletion of saved bits. Depending on the drive, the data recovery specialist might be able to recover some data. Recovering data from defect Hitachi drives is usually impossible, since the manufacturer does not provide any kind of firmware information. Various other products allow at least reading data from other platters.
Strange but true: even internal imbalances can cause trouble. Irregular data platter rotation due to broken bearings will result in unwanted track changes of which the drive is not aware; the result is sometimes total data hodgepodge. Bearing failure is usually the result of improper handling during transportation of the drive. The data recovery specialist will often be able to recover saved data by balancing the drive (slightly shifting and centrally rearranging the platters).
Logical mistakes make up 40% of Kroll Ontrack's assignments; luckily, they do not require processing in the clean room. Usually, those mistakes are accidental misuse or deletion of files, where the drive itself has no malfunctions. Even if the problem lies within the drive, it might be a simple defective sector and nothing more severe. Every drive has defective sectors: a primary list will carry a list of bad sectors from the factory; the secondary, so called growing list, will be updated in the course of operation if more sectors become useless. This occurs rather often and is normal, but if your hard drive S.M.A.R.T. feature tells you that you better save some data fast, you're in trouble. This often means that the growing list is full and an above-average number of sectors are dying.
Now let's talk about an irreversible defect. Your hard drive will die if fire or another heat source heats it up beyond the Curie temperature of the magnetic material of your drive; this cases the magnetization to be neutralized completely. The Curie temperature depends on the material used in a hard drive; magneto-optical devices use with Curie point principle on purpose (heating via laser), but hard drives do not.
A user who tries to solve issues by himself can make things worse - according to Kroll Ontrack, this is the biggest problem with data recovery. According to statistics, Europeans are worse than Americans in trying to solve problems on their own. Situations also turn bad if, for example, administrators are afraid of their bosses' reactions. Once, someone even sent a floppy drive instead of the defective hard drive - this is a clear indication of pressure placed on the person involved.