What is the formula for working out the final RAID size of a RAID 5 array knowing the number of disk and the size of each disk?

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Therefore, the usable capacity of a RAID 5 array is (N-1) x S(min) , where N is the total number of drives in the array and S(min) is the capacity of the smallest drive in the array.

Add the number of hard drives minus 1. Either all the drives have to be the same size, or if you use different sizes, the RAID will treat all the drives the same as it would the smaller one.

10 1TB hard drives equal 9TB of storage.

9 1TB drives and one 500GB drives is 4.5TB of storage (each 1TB is treated as a 500GB)