International Research Deck

Raid 0 can only work with 2 disks. You would need raid 5, which motherboards do not generally support very well. You either need an expensive raid controller card, or could use software raid, which currently is only available as standard on Linux.
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You can make a RAID 0 or 1 out of any number of dscs (the minimum is 2). The size doesn’t matter. Where the hell did you pull 7.5 TB from?

2x250GB in a RAID 0=500GB, but know that the data is striped, so losing either disc means losing everything. I strongly recommend you investigate RAID 5.
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Yes… if your motherboard supports RAID0 or if you have a controller card. RAID0 requires at least two drives of equal size and will create a single combined space drive for you. So, if you have 2 10GB hard drives in RAID0, this will give you 20GB space as a single drive. If you have 3 10GB in RAID0 then you have 30GB space. Etcetera.

As stated, the motherboard must support this feature or you need to buy a RAID expansion card (which can be expensive).

Also note that I have successfully creaded a RAID0 with two drives of unequal size. One was a 200GB and the other was a 250GB and because I put them in RAID0 the controller card limited the 250GB drive to a 200GB to match the smaller. I don’t recommend this, but it does work.
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Ghowriter