Several hard disk drive manufacturers have come under fire for quietly replacing some HDD models with those that use a technology called singled magnetic recording (SMR) instead of traditional magnetic recording (CMR) SMR drives generally perform poorly, especially in RAID environments . For this reason, two lawsuits were filed against Western Digital, which incorporated some of the SMR drives into its Red HDD lineup sold for network attached storage (NAS) appliances. In response, WD rebranded its Red series and finally became more transparent about what recording technologies its HDDs use.

There is quite a bit to unpack there, but it is best to start with the recording technology at the heart of the fuss: CMR, formerly known as perpendicular magnetic recording (PMR), is an arrangement of magnetic tracks next to each other, according to Synology's description, Writing data to a CMR drive works by "aligning the poles of the magnetic elements, which represent bits of data, perpendicular to the surface of the disk."

SMR is a somewhat newer technology that partially overlaps with track. A common analogy is that of a roof shingle. Drive manufacturers have moved some of their HDDs to SMR technology, presumably because the tracks can be thinner, allowing for higher data densities and higher capacities to be produced more cheaply. However, there is a tradeoff. When data is written to one track, bits on overlapping tracks are affected and they must be rewritten. Rewriting existing data has a negative impact on performance.

While this is not a major issue in some cases, it can have a significant impact in a NAS environment; ServeTheHome staff tested this in a direct match between the CMR and SMR Red drives. [The largest performance difference was observed in the 125GB file copy test and the RAIDZ resilver benchmark. [All three CMR drives comfortably completed the resilver in less than 17 hours, while the SMR drive took nearly 230 hours to perform the same task," ServeTheHome said.

As discovered by our friends at Tom's Hardware last month, WD has been hit with two class action lawsuits regarding the use of SMR on some of its drives. One of the lawsuits alleges that Western Digital "secretly downgraded its Red NAS hard drives from standard CMR technology to the much slower SMR technology without disclosing it to customers."

Joel Hruska of ExtremeTech sued Seagate and Toshiba for similar misconduct, but is unaware of any related lawsuits having been filed against either company.

The controversy goes back further, to April, when it was first discovered that WD and others were using SMR technology without necessarily being upfront about it; in an April 23 blog post, Western Digital said that "concerns were clearly heard" and that it was not clear which capacities used CMR and which HDD families, outlining which capacities use CMRs and which capacities use SMRs.

Following this, Western Digital added a new "Plus" tier to its Red drive product line and rebranded it. This allows customers to choose from Red, Red Plus, and Red Pro drives. Only regular Red drives use SMR.

According to Western Digital, the Red drives are fine for "light SOHO (small office/home office) workloads," but it is a bit disappointing that all drives still use SMR.

"Our use case analysis shows that SOHO workloads are typically based on short access times to the drives. This results in a very low average throughput (compared to the available throughput of the drive) and allows the DMSMR drive plenty of idle time to perform the necessary background processing, making it an ideal fit for this application," said Western Digital .

Unless there is a significant price difference, the CMR model is recommended for NAS boxes.