RAID requires identically sized hard drives.
RAID may hard drives faster. With 2 drives, you can either do RAID 0 where the system writes data alternately to both drives (speeds disk access) or RAID 1 where the data is copied to both drives (slower).
Then you have RAID 5 and higher where the data and error checking are spread across several drives. The whole point of RAID 5 is that when one drive fails, you can replace that drive and the whole system rebuilds itself. The disadvantage is that you effectively lose one drives' worth of storage. For example, a RAID 5 array of four 1 terabyte drives can only store 3 TB's of data.
You can do RAID's two ways:
1. It's built into many motherboards. Advantage - cheap. Disadvantage - not really portable; you will need another motherboard with an identical controller.
2. A dedicated RAID card. Advantage - fast and portable; you simply transplant the drives and card to a new computer. Disadvantage - more expensive.