Western Digital launched the WD Red line of hard drives and solid state drives specifically for NAS applications. The rigors of NAS involves not just near 24x7 uptime, but also the ability to work in RAID volumes, as most NAS servers ease the process for end users to set up RAID volumes for data redundancy. Data Storage-focused tech publication Blocks & Files alleges that some WD Red HDDs are shipping with shingled magnetic recording (SMR), a physical-layer data recording technique that makes the drive unfit for RAID, and in turn unfit for most serious NAS setups.

An elaborate investigation and discussion in a Smartmontools thread reveals that both Western Digital and Seagate appear to be shipping HDDs with "DM-SMR" (drive-managed SMR), a feature where the HDD's controller internally performs SMR to increase data density when the physical media is running out of space under CMR (conventional magnetic recording, with conventionally spaced tracks). Think of this as a crude analogue to pseudo-SLC caching employed by modern SSDs. Making matters worse is that DM-SMR drives don't report SMR to host controllers. "Beware of SMR drives in CMR clothing," the thread's title reads. A thread in the user forums of Synology, a prominent NAS manufacturer, recounts a similar horror story.