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[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Performance, Quality and Reliability | Redundant Arrays of Inexpensive Disks (RAID) | RAID Concepts and Issues | RAID Reliability Issues ]
Under normal circumstances, if a hard disk fails, its data is no longer accessible. Most failures, especially mechanical ones, do not actually cause the data on the platter surfaces to be erased, however. This means that with special procedures, it is possible to retrieve most, if not all, of the data on a hard disk--if it is important enough to justify the expense. This process is called data recovery and is discussed in general terms in this section.
RAID, particularly RAID that uses striping, complicates data recovery significantly. The data is no longer placed in relatively simple fashion on a single hard disk, but rather, distributed across many drives. Despite this complication, it is possible to do data recovery in most cases; it just costs more due to the complexity of the array. For example, there have been cases of two drives failing on a RAID 5 array and the data being successfully recovered.
That said, it should be remembered that compared to a single drive, a RAID array that includes redundancy provides fault tolerance that means in most cases a drive failure won't result in any lost data at all. If a fault occurs, and it is dealt with quickly and competently, in most cases the need for data recovery never arises. In fact, some companies refer to rebuilding a replaced failed drive in a RAID array as "real-time data recovery", which I suppose it is, in a way, though I personally find that term a bit confusing.
Next: RAID Levels