RAID, by definition, requires multiple drives. The minimum is 2. The two most common configurations are RAID 0 and RAID 1. RAID 0 (zero) is used to improve performance by adding more heads to the array to write and retrieve data. It's the same as your using both hands for a task as opposed to one. RAID 1 is also called "mirroring". Two drives have the same data written to both with the idea that, if one drive fails, the other still holds the data.
SATA drives each have their own cable which connects to the interface. IDE can connect 2 drives on each channel. SATA RAID will use two channels on your MB or add in card. IDE RAID can connect 2 drives to each channel. Other RAID array configurations exit as well.
Can you run just one serial HDD, or must you use two and set up raid?