Depending on your requirements you should at least take a look into RAID 1, and if its for harddrive redundancy ie if one fails the other one is still ok then the computer stays up you may like some extra hardware called a raid controller, but you may already have one or it might be possible to do it using software raid, i dont know as i've not used software raid but i use raid all the time, normally raid 5 but a raid 1 or raid 0+1 is more for you probably
cheers
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