I will clarify the RAID part.
RAID 0, min 2 disks- also known as stripe, you connect 2 or more IDENTICAL drives and they become known as one to the computer. For example, two 250 gig hard drives would show up as 1 500 gig to the computer. This improves performance and obviously storage capacity. Word of caution, if one hard drive fails, the data on any others are useless. This is because bite are written not in series on one drive, but in series across all the drives. For example, bit 1 goes to disk 1, then bit 2 goes to disk 2, then bit 3 goes back to disk 1, etc. You kind of put all your eggs in one basket when you do this... Your probability of one drive failing is now doubled because you have two drives, and again, if one fails, both are useless.
RAID 1 - Also known as mirror, you connect 2 IDENTICAL drives and they act as one, but the second drive is a mirror image of the first. Every byte that is written to the first drive, is written to the second. This is great for protecting your data from hard drive failure, and not having to worry about doing backups. If one hard drive crashes, just pop the other on in it's place.
There are ways to get like 4 hard drives and do a RAID 0+1 which will give you the better of both worlds, but obviously cost a lot. For example, if you have 4 250 gig hard drives, in RAID 0+1 your computer would only see 1 500 GIG drive, the other 2 disks are mirrored of the other 2. So you have speed, reliabilty, and extra storage.
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RAID - Wikipedia, the free encyclopedia