Simple:
If you have two HD's in your computer, you can set them up in raid 0, raid 1 or raid 10 formation. Raid makes two HD's act as one. If you set them up in Raid 0 formation (striping) then they will gain a nice performwnce boost. Setting them up in Raid 1 makes them much less likely to corrupt or lose data, and RAID 10 does both.
- I don't write Tom's Hardware Guide, I just preach it"