Well, I have my computer set up in RAID 1, which mirrors data from one hard drive to another.
Windows sees you only have one drive, when there is in fact two working in tandem.
I find it adds a layer of safety since I've had bad luck with hard drives in the past...
I created my RAID array before installing Windows however, so even though it is probably possible to install RAID after installing, I'm finding it very hard to find the information