March 2 2015: Redundant Array of Independent Disks  or RAID is a standard technique followed by enterprise data centres, where  data is saved simultaneously  on two or more separate  hard drives,  so that if one fails, another kicks in. 
With personal storage  exceeding 1-2 terabytes these days,  our own file are no less valuable  -- and  WD ( Western Digital)  has just launched  the first portable, external dual-drive storage and backup solution, the  MyPassport Pro  for Mac systems. 
The tough aluminum casing encloses  2 drives delivering  2TB ( Rs 24,000 ) or 4 TB ( Rs 33,000).  The Mac's Thunderbolt connector ensures high speed data transfer  at over 230 MBPS -- twice as fast as a USB3.0   cable.
More details here