The Raspberry Pi does not have a SATA controller on board, and there is no way to attach extra hard drives except via USB ports. The cool thing is that you can get a USB RAID controller. This makes file storage on the Raspberry Pi a very attractive option. Technically, the peripheral needed here is called the USB SATA multiplier.
There is no one-step guide to configuring these multipliers. Some need to be configured using software in Windows or Linux, while others may have DIP switches that configure the multiplier to a specific configuration.
Addonics is a well-known and fairly easy-to-source multiplier. You will need to search around on the Internet, online auctions, and shops for it. The prices are around the same as the Raspberry Pi but if you are looking for redundant storage, then these are the cheapest options you will find.