Is the WD Red 4TB SMR or CMR?

The short version: the plain WD Red 4 TB is SMR; WD Red Plus and WD Red Pro 4 TB are CMR. Western Digital split the Red line specifically over this, so the model number matters.

The model numbers

• WD40EFAX, the original plain WD Red 4 TB, is SMR (shingled).
• WD Red Plus 4 TB (WD40EFPX / WD40EFZX): CMR.
• WD Red Pro 4 TB: CMR, 7200 rpm, higher workload rating.

Western Digital documents the recording technology per model on its support pages: always confirm the exact model number printed on the drive rather than relying on the "WD Red" name alone.

Why it matters for a NAS

SMR drives overlap their write tracks and must rewrite whole zones on change, which collapses their sustained write speed during the continuous writes of a RAID rebuild. That can turn a rebuild from hours into days, or stall it: the reason the plain WD Red's SMR caused problems in NAS arrays. For any NAS, choose WD Red Plus or WD Red Pro (both CMR).

What to buy

The table lists in-stock WD Red Plus drives, all CMR, ranked by price per terabyte. WD Red Plus is the drive most people mean when they say "WD Red" for a NAS.