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…i want a wd raptor for my os and another 200gb sata to store everythin else, does that make sense?
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In terms of a RAID, no, it doesn’t make any sense. One of the key points about a RAID is that your OS would see it as one volume.
Addressing your question about having different drives in an array, it’s always best to have as similar drives as possible for best performance and stability.