CMR vs SMR: what's the difference?

For a home NAS or a hard drive you’ll regularly fill, change, and reuse, I’d buy CMR. SMR can make sense for a cheaper disk that mostly holds files you leave alone, provided you can tolerate slower writes. The difference is how the drive fits data onto its disks, and how much work it has to do when that data changes.

You don’t need to avoid every SMR drive. You do need to know which kind you’re buying before putting it into a job where write performance matters.

What CMR and SMR actually mean

Inside a hard drive, data is stored in narrow circular tracks on spinning disks called platters. Conventional magnetic recording (CMR) keeps those tracks separate, so the drive can rewrite one without having to rewrite its neighbours.

Shingled magnetic recording (SMR) overlaps tracks like shingles on a roof. The part of each track left exposed is enough to read, and packing tracks closer together lets the manufacturer fit more data onto the same surface.

The complication comes when you change existing data. Rewriting a track can disturb the overlapping tracks beside it, so the drive has to preserve and rewrite more data than you actually changed. Tracks are grouped into bands to keep that work within a manageable area. Seagate’s explanation of shingled recording shows how the overlap and rewriting work.

Why a file copy can start fast and then slow down

An SMR drive doesn’t necessarily feel slow when you save a few files. Drive-managed models handle the extra work internally, and idle time lets them catch up on background tasks. Keep sending writes without a break and commands can take longer to finish.

That makes the first few moments of a copy a poor guide to how the disk will behave through a long transfer. If you regularly replace large folders, move several terabytes at once, or need a backup to finish in a limited window, I’d choose CMR for more predictable writes.

Reading existing files doesn’t require the same rewriting. SMR still supports reading data from different parts of the disk, so a write slowdown doesn’t mean it will be equally slow at playing a stored movie or opening an archived photo.

For a NAS, start with CMR

A NAS, or network-attached storage box, shares files across your home network. Even if you mostly use it to watch movies, buying drives also means planning for the day one fails.

RAID combines drives to provide storage and, in some layouts, keep working after a disk failure. Rebuilding that protection writes data to the replacement drive. That workload matters just as much as your everyday file copying.

I’d use CMR in a home NAS array and check the enclosure maker’s compatibility list before ordering. This is especially true for ZFS, the storage system used by TrueNAS.

When SMR is a reasonable buy

A separate disk holding finished video projects, older photos, or a media collection can be a reasonable home for SMR. The useful distinction is that you copy files over and mostly leave them there.

I’d consider it when the saving is meaningful and waiting for a long copy won’t cause a problem. If the CMR alternative costs only a little more, the flexibility to reuse it for other jobs is worth paying for.

A backup drive needs a closer look. An occasional copy you keep offline is different from a disk that constantly adds versions and removes old ones. For frequent backups or a tight overnight schedule, I’d choose CMR.

For most buyers, CMR is the easier choice: it leaves you fewer constraints when your storage needs change. Buy SMR deliberately for a job that suits it, with a discount that makes the tradeoff worthwhile.