CMR vs SMR Hard Drives: What to Put in a NAS

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Conventional Magnetic Recording (CMR) and Shingled Magnetic Recording (SMR) are two ways hard drives write data to their platters. For everyday storage you might never notice the difference. In a NAS, especially during a RAID rebuild after a drive failure, it can matter a lot.

This became widely known in 2020, when some NAS-branded drives were found to use SMR without clear labeling. Manufacturers now disclose recording technology more clearly. Here's what you need to know to choose correctly.

How CMR works

CMR writes data in separate, non-overlapping tracks. Any track can be rewritten independently, so random writes and rewrites stay fast and predictable. This consistent behavior is why CMR is preferred for NAS and RAID use.

How SMR works

SMR overlaps tracks like shingles on a roof to fit more data on each platter. It's cheaper per terabyte, but rewriting data means rewriting neighboring tracks too. SMR drives hide this with a fast cache area. When the cache fills during large sustained writes, performance can drop sharply while the drive reorganizes data in the background.

Head-to-head

Why SMR causes trouble in a NAS

When a drive in a RAID array fails, the NAS rebuilds its data onto a replacement by writing huge amounts of data continuously. That's exactly the workload SMR handles worst. Rebuilds can take far longer, and during that time the array is vulnerable to a second failure. Some ZFS users reported SMR drives slowing so badly that they were dropped from the array. That's why NAS makers and enthusiasts recommend CMR.

SMR isn't bad everywhere. For a single external backup drive or an archive you write once and mostly read, it's cost-effective and fine.

How to tell which a drive uses

• WD Red Plus and WD Red Pro: CMR. WD Red (non-Plus): SMR.
• Seagate IronWolf and IronWolf Pro: CMR.
• Toshiba N300: CMR.
• Enterprise drives (WD Ultrastar, Seagate Exos, Toshiba MG series): generally CMR, and popular for larger NAS builds.
• Always check the manufacturer's datasheet for the exact model number, and check your NAS maker's compatibility list.

The verdict

For a NAS or any RAID setup, choose CMR. The extra cost buys predictable performance and safer rebuilds. Use SMR only for single-drive archives and read-mostly backups. Need a NAS? See our best 2-bay and 4-bay NAS.

What about SSDs in a NAS?

SSDs sidestep the CMR/SMR question entirely. They're silent, fast and very good for NAS use, and many NAS units let you use an NVMe SSD as a cache in front of hard drives. The catch is cost per terabyte: for multi-terabyte media libraries and backups, hard drives are still far cheaper. A common approach is hard drives for bulk storage, with an SSD cache or a small SSD volume for apps and frequently accessed files. Check your NAS's support for M.2 cache and volumes before buying.

How big should NAS drives be?

Buy for two to three years of growth, not just today's needs. In a mirrored 2-bay setup, usable space equals one drive, so two 8TB drives give you 8TB. In a 4-bay RAID 5 or SHR setup, you lose roughly one drive's worth of capacity to protection. Larger drives cost less per terabyte, but they take longer to rebuild after a failure, which is one more reason to choose CMR and keep an offsite backup.

A quick checklist before you buy NAS drives

• Confirm the drive is CMR on the maker's datasheet for the exact model number.
• Check your NAS maker's compatibility list for that model.
• Buy drives from a retailer with a clear return policy, and consider buying from different batches to reduce the chance of simultaneous failures.
• Run the NAS's extended drive test before trusting new drives with your data.

CMR drives we recommend for a NAS