One advantage of RAID 5 over RAID 1 mirroring is that with a RAID 5 hot spare, 2 drives can fail and you don't lose the data which is not possible with 2 RAID 1 mirrored drives.  However RAID 5 is faster.  Another advantage is that you have to buy double the disks for RAID 1 as compared with RAID 5. 
 
Chuck

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