Hi!
So having multiple Hard Drives (HDD) will enable you to do backups or some people like to have 2 HDD in order to put their games on the drive that is not being used by the Operating system in order to get a little bit more performance.
As far as installing an OS on one drive and storing everything else on another drive. There is a lot of uses for that, for example I had my Vista machine and when I switched to Windows 7 I didn't have to do any backups since all my Data/Games were already on a drive ready to go as soon as I entered my newly installed OS. So it's really handy when you want to re-install an Operating System to have all your data on another drive, you can skip the backup part.
RAID is a way to organize drives in order to gain either more performance, more safety or in some cases both. The thing you need to know about RAID is that their is 2 Major kind: RAID0 and RAID1.
RAID 0 will let you use the writing and reading power of 2 seperate HDD into one storing volume so you get theoredicaly twice the speed but you also have twice then chance of failure.
RAID1 on the other hand is also known as drive mirroring so everything you write to a single drive will be simultaniously written on the other one for added safety.
There is also RAID5 which is more used by servers it's another kind of RAID0 but requires at least 3 drives and it has a lot of neat features like per say one of the drives goes down, you can plug a fresh one right in and all the data that was on the dead drive will be written on the new drive and everything will be as it was before the failure.
And lastly there is RAID10 which is simply 2 RAID1 volumes in RAID0, it requires atleast 4drives to functions.
Hope this helps and answers all your questions,
Cheers.
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