It's simple. You need multiple hard drives to create an array. You can do striping for performance and mirroring for reliability. If you have enough, you can even do both.
Raid 5 is the best option for price and reliability. It involves using a miniumum of three hard drives and the data is spread across the discs in stripes but also adds parity checking. This allows one disc to go down without loosing your data. You can then replace the damaged disc and the RAID system will rebuild the new disc using data stored on the other discs.