I've seen the word RAID used in conjunction with hard drives. Six types of RAID have been defined named Level 0 to Level 5, all involving different data storage schemes.
RAID storage divides data across two or more drives for faster read and write times.
Mirroring protects data by storing at least one extra copy of the data on a separate disk in the array, writing all data at least twice. This method is safe, but not as fast as the other methods. If a data drive fails, the system can recalculate the equation to reveal the missing bits. But if two drives fail, not enough information remains for the parity scheme to recover your data. Replacing a drive in case of failure is called swapping.