According to Golem:
"According to Western Digital, only the WD Red with 8 TByte to 14 TByte use the classic Perpendicular Magnetic Recording, while the 2 TByte to 6 TByte models use the slower Shingled Magnetic Recording" - these have a significantly lower transfer rate for random accesses. The slower SMR models are labelled (PDF) as EFAX (256 MByte cache), while the faster PMR drives are labelled EFRX (64 MByte buffer)."
Source: https://www.golem.de/news...