Personally, I see the WD Red as a consumer grade HDD for NAS, leaving the unlucky desktop features of desktop HDD away. Of course, WDC does not want to torpedize thier bread-and-butter enterprise business - so they set a radnomly defined marketing boundary.
The question you have to answer: Do I need enterprise grade HDD (slightly lower MTBF, better warranty terms), or can I go for consumer grade disks (lower cost, shorter warranty)?
The decision should be done based on the effective usage of the NAS. A sytem that is fully busy 24*7 (ie. serving business, data center-like, massive online backup, serving virtual machines where uptime is critical) should run on enterprise drives. A sytem for home usage, some sismple 8..10 hour office applications, do some download and playback music and video for some hours can perfectly run on conumer drives - calculate one or two spare drives. And then - a geek who is planing to upgrade the HDD to 4 TB or even larger drives when available and affordable within the shsorter warranty time (typical thee years)...?
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