There is the Raid 1 system where identical data is stored on two hard disks (100 percent redundancy). When one disk drive fails, all data is immediately available on the other without any impact of performance or data integrity. This is called "Disk Mirroring". This is typically used by day-traders who can't afford to have their system do down during the day due to a hard disk drive failure.
The Raid 0 system combines two hard disk drives in a way that the data coming from the user is cut into manageable blocks. There blocks are stripped across the different drives of the Raid 0 array. By doing this, two or more hard drives are combined and the read/write performance, especially for sequential acess can be improved. However no redunacy information is stored in a Raid O array, which means that if one drive fails, all data is lost. This type of system is used by backtesters who want the maximum speed to reduce the time of backtesting which can in many cases be several hours.