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From Mediacenterhouse

RAID is a method of configuring multiple hard drives to act as one, reducing the probability of data loss in case of drive failure. RAID is implemented in either software (where the operating system knows about all connected drives and actively maintains them itself) or hardware (where a special controller makes the OS think there's only one drive and maintains the drives 'invisibly')

There are several versions of RAID you can implement.

• RAID0 = Striping. This writes different bits of data across the drives you have in your RAID array speeding up read/write times considerably. However because your data is spread across different drives, if one drive were to fail you would lose all your data!

• RAID1 = Mirroring. Data is stored on one hard disk and that disk is 'copied' onto the others in your array. In the even of a drive failing, you have the others in the array to take its place.

• RAID5 = Striping with parity. This does the striping across drives as in RAID0 but holds one drive for parity (i.e fault checking). It needs 3 drives minimum and for an example lets say that it writes the number 4 to drive one, then the number 5 to drive two, then on drive three (parity) it'll write 9 (a sum of the other two). So if any one of those three drives fail it can work out what the missing digit is and rebuild it all once a new drive has been connected. It requires all drives of equal size and is a fault tolerant way of achieving the data speeds of RAID0. It is hardware intensive though.