RAID
RAID Level 5 is what is probably used most of the time. At least three hard disks are needed to build a RAID 5 storage array. Each block of data will be stored in three different places. Two of these places will store the block as it is, the third will store a checksum. This checksum is a special case of a Reed-Solomon code that only uses bitwise addition. Usually, it is  calculated using the XOR method. Since this method is symmetric, one lost data block can be rebuilt from the other data block and the checksum. For each block, a different disk will hold the parity block which holds the checksum. This is done to increase redundancy. Any disk can fail. Overall, there will be one disk holding the checksums, so the total usable capacity will be that of all disks except for one. The size of the resulting logical disk will be the size of all disks together, except for one disk which holds parity information.