RAID is a Redundant Array of Inexpensive Disks . Therefore, it is hardware - two or more disk drives.
Your disk drive is a mechanical device. I read somewhere the average life expectancy of a disk drive is about 4 years. When it fails, expect everything on your disk to go bye bye.
If you value what's on your hard drive, you should either implement RAID or backup or both.
RAID is usually used in servers. Not many people have it on their home PC's. However, IDE (most home hard drives are IDE) RAID is becoming more common. SCSI is mostly used in an enterprise setting - ie. business.
Some OS' can do what is called "software raid" meaning that the OS takes care of writing to the disk drives. However, most professionals would recommend using "hardware RAID" meaning you use a special "RAID Controller" hardware card to manage the disk Input/Output (IO) process.
There are different levels of RAID describing different schemes - the most commonly referred to are:
RAID 0 - Mirroring.
RAID 1 - Striping
RAID 0+1 or "10" - Mirroring and Striping
RAID 5 - Striping with Parity.
RAID 5 is usually preferred in a business/enterprise setting because it will allow 3 or more drives to continue to operate, even though one has failed, without data loss.
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