Be careful about the items that you're comparing. The hard drives used in the consumer market are not the same...
devices that are used in enterprise storage arrays.
Enterprise-class hard drives are designed to offer performance and reliability that are better than consumer-class drives. This, of course, drives up the cost of enterprise drives and inflates the cost of enterprise storage arrays. For example, SATA enterprise or desktop disk drives that have gone through more exhaustive testing and burn-in are also combined with dual-porting cards for availability -- adding to the cost of a drive.
But the cost of an array goes far beyond the hard drives. Storage arrays also demand power supplies, cooling components and controllers, many of which are redundant. Software design and testing also has to be included. And all of the design/engineering time that went into the storage array has to be recaptured over time. Storage arrray vendors also have to make a profit to stay in business and continue to invest in R&D, sales, marketing, etc.
From a theoretical standpoint, you could go off and buy the drives and other parts, write your own software/firmware and then stitch it all together to form your own storage array. But why would you want to unless you have a lot of free time and resources at your disposal?
Go back to the beginning of the Disk Hardware FAQ Guide.
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