I'm not sure if you picked up on this or not ... but RAID 1 is used to have another copy of stuff just in case one hard drive fails like northbridge said. However, you should also note that in RAID 0 since you are writing parts of files to each drive, if one drive fails you will lose all of your data. It is also adjustable how much of each file is split .. EG:
you can make 16 kb blocks which means that each drive will be read/written 32 kb at a time (16 from each hard drive). For video and such they use larger blocks, this is just an example.
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