What is RAID, why do you need it, and what are all those mode numbers that are constantly bandied about? RAID stands for "redundant array of independent disks," and you may or may not need it depending on your data-storage requirements.
The biggest gain from using RAID is protection against drive failure, which, according to Google and other experts, happens a lot more often than hard-drive manufacturers like to admit. Note that the word array is included in the acronym, so saying "RAID array," as a lot of people do, is redundant. Clearly, storage folks have a strange sense of humour.
In the old days, when the fastest and largest hard drives carried a very heavy premium (faster drives still do, though not nearly to the same degree), RAID was created to combine multiple, less-expensive drives into a single, higher-capacity and/or faster volume. Redundancy, also known as fault tolerance or failover protection, was included so that the loss of one drive wouldn't render an entire array and its data useless.
As such, RAID has several levels, or methods by which the drives are ganged together, with data distributed across the drives. The RAID levels are commonly referred to by number. The three most common levels in the consumer and small-office markets are RAID 0, RAID 1, and RAID 5, which I'll cover first along with other common options such as JBOD ("just a bunch of disks"), Microsoft's RAID-like Drive Extender and RAID-virtualisation technologies such as those from Drobo, Netgear, Synology and Seagate.
Most RAID modes don't require that you employ drives of equal size, but they'll use only the capacity on each drive that equals the capacity of the smallest drive in the array. That is, if you mix a 500GB drive with a 1TB drive, the setup will treat both as 500GB drives.
Keep in mind that RAID's data redundancy is a hedge only against data loss due to drive failure, and a way to keep you working until you can replace the bad drive. RAID offers no protection against data lost to malware, theft, or natural disaster, and it's certainly no substitute for proper backup practices.
Common RAID Modes
Picture the 0 in "RAID 0" as a race track, and you're on to its primary purpose, speed. RAID 0 distributes data across multiple drives (for example, block A goes to/from drive 1, block B goes to/from drive 2), which permits the increased write and read speeds. This approach is sometimes referred to as striping, and other modes (as you'll see later) employ the technique as well.
Always remember, however, that RAID 0 offers zero protection against drive failure, as no duplicate or parity information is written. Hence, when a drive fails, you end up with a puzzle that's missing pieces. In such a situation, your data is lost, unless of course you want to spend a very large sum of money trying to recover it.
RAID 1 creates a mirror of your data across two drives, that is, the array writes and reads the very same data to pairs of drives. The drives are equal partners. Should either fail, you can continue working with the good one until you can replace the bad one. RAID 1 is the simplest, easiest method to create a failover disk storage subsystem. It costs you a whopping 50 percent of your total available drive capacity, however, for example, two 1TB drives in a mirrored array nets you 1TB of usable space, not 2TB.
You may have as many pairs of mirrored drives as your RAID controller allows. And in the unlikely event that said consumer grade data traffic cop supports duplex reading, RAID 1 can provide an increase in read speeds by fetching blocks alternately from each drive.