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Raid 1 provides fault tolerance from disk errors and failure of all but one of the drives. Increased read performance occurs when using a multi-threaded operating system that supports split seeks. Array continues to operate so long as at least one drive is functioning.
If one hard drive fails the 2nd hard drive will continue running without loss of data, the faulty drive can be replaced and the array will be rebuilt.
Raid 0 provides improved performance and additional storage but no fault tolerance, both drives are linked to make one big drive giving much better performance than a standard single drive.