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
