Redundant Array of Inexpensive Disks is the full form of RAID. It is also known as Redundant Array of Independent Disks because it is all about using two hard drives at a same time to achieve certain degree of reliability and data storage, of course.
Actually, RAID is used to make two or more hard drives act as single logical unit and this is done with the help of some hardware or software. Software and hardware is used in a way that operating system takes these set of drives as a single unit while transferring and recovering data from any of them. It is worth mentioning that there are different RAID levels that are available today. However, these levels start from 0 to 5 and known as the most basic ones.
When you use hardware to manage RAID subsystem, then the concept of SCSI RAID comes into the picture. When you use hardware RAID, it helps to span RAID Subsystem over multiple SCSI channels. Actually, different RAID subsystem is connected to the host using SCSI controllers and that makes RAID subsystem to look like a single logical unit