1 – 1/n
That is the formula to compute the amount of available storage space for a RAID 5 disk array which is what I’m now thinking of getting for home.
The lowest number of drives that can be used for a RAID 5 array is three, so that would mean 1 – 1/3 = 2/3 So if we had a RAID 5 array of three 4TB drives that would mean the amount of available storage in the array would be 12TB x 2/3 = 8TB. For an array of four 4TB drives the available space computes to 12TB.
No one likes to think of purchasing 12TB or 16TB of storage space and then giving up 4TB for the sake of an array, but the nice thing is that one gains a substantial increase in both data security and in performance for that cost.
The array in question, which seems to be the best available on the market based on my searching, is the StarTech SAT3540U3ER.
The critical features are that it provides hardware RAID, meaning the box just does the RAID for you, independent of whatever computer(s) it may be connected to (unlike software RAID which is setup in the operating system of a computer attached to the drives). It also has USB 3.0 and eSata interfaces for the highest available speed connection to computers. Finally, it supports hot-swapping, another critical feature whereby drives can be swapped in or out of the array while its running live.
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