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RAID is simply an acronym meaning 'Redundant Array of Independent Disks'.
The different configurations are built using a hardware-based controller and
enable a pair of disks to be used in a variety of ways...
RAID 1, RAID 0, SPAN or JBOD?
RAID 1 is the choice for maximum protection. Both drives are combined into one volume and identical data written to both. Data is mirrored from one drive to the other. In the event of one drive failing all your data remains safe on the other.
RAID 0 again combines both drives into one volume that is twice as big. So if two 750GB drives are used you would get 1.5TB of storage. The advantages here are speed and space. Data is read and written across both drives but you get no protection. If one fails you lose all the data.
SPAN combines the two drives but allows you to fill one drive first and then it switches to the other. You get twice the space like RAID 0 but in the event of one disk failing, you only lose the data on the failed disk.
JBOD means 'just a bunch of disks'. This gives you two separate drives in one enclosure that work independently of each other. This is the only option that appears as two separate volumes on your desktop and you use them exactly like any other attached volume.