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CMR vs. SMR Hard Drives: What’s the Difference?

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Since 2015, hard disk manufacturers have produced a new type of drive: SMR, which stores more data per disk but comes with some drawbacks compared to the conventional storage method, called CMR. Each type has pros and cons—let’s take a look.

Two Different Ways of Storing Data on a Disk

Hard drives store data in “tracks,” which are circular paths typically oriented in concentric rings on the top and bottom surfaces of a hard disk platter. Each hard disk unit can contain multiple platters, which allows the drive to store more data.

Historically, manufacturers have increased storage capacity in hard disk models by either increasing the number of platters in the drive or increasing the write density on the disk. In the past, the circular tracks written to the disk never overlapped. The data storage industry calls this “Perpendicular Magnetic Recording” (PMR) or “Conventional Magnetic Recording” (CMR).

A track diagram of CMR vs. SMR in hard drives
A track diagram of CMR vs. SMR in hard drives. Seagate

Recently, a new technique for increasing write density called “Shingled Magnetic Recording” (or SMR) has emerged. SMR drives write data using a special method that partially overwrites previously written tracks on a hard disk platter. The manufacturers use the analogy of roof shingles that partially overlap each other to explain this technique, which is where the “shingled” part of the name comes from.

While SMR drives increase capacity for lower cost (because the drives can use fewer platters than a CMR drive at the same capacity), the way they work also comes with a speed penalty. When you copy data to an SMR drive, the drive temporarily stores the data in a special cache area and uses idle time later to organize it into shingled regions on the platter. Long, sustained writes suffer speed penalties because if the cache fills up, each time an SMR drive overwrites part of a previous track, it must read and re-write the “partially covered” underlying data as well. So SMR drives can perform dramatically slower than CMR drives.

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SMR’s slow performance led to a controversy in 2020 and 2021 when people realized that manufacturers were selling SMR drives without labeling them (in both external hard disks and internal drives), arguably selling an inferior product without warning customers. Some of these complaints even led to a $2.7 million class-action settlement with Western Digital in 2021.

So Is SMR Bad? Which Drive Type Should I Choose?

Whether SMR is bad or not is a subjective call. If you want massive amounts of storage for less money and you don’t mind the performance penalties involved, an SMR drive might be an OK choice for you. Typically, a 16TB SMR drive costs less than a 16 TB CMR drive, for example. If you use a drive for occasional single-disk backups, an SMR drive might be fine.

But beware: A major drawback of SMR found in testing by ServeTheHome is that using a slow SMR drive in a RAID array can put the entire array’s data at risk for longer because it takes far longer for the SMR drive to get integrated into the array. So using SMR drives in a NAS with multiple disks is probably not a good idea.

Luckily, some manufacturers such as Seagate have begun publishing data online that clearly shows which drives in their lineup use SMR or CMR technology, although most still do not clearly label the drives as SMR or CMR when you purchase them from a retailer. Still, with this data in your hands, you can make a more informed purchasing decision—such as selecting a CMR drive to use in a home-built external USB drive.

Overall, we recommend purchasing CMR drives whenever possible due to the performance issues with SMR drives. But it remains a personal and budget choice depending on your situation and how you use the drive. Good luck!

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