SSDs are too expensive per GB, and far too performant to waste on home videos and PowerPoint presentations, but that doesn’t mean the specs of the drives you buy don’t matter. The thing is, while the common specs may make two hard drives seem similar, there are several more obscure specifications that tell a different story.

SMR vs CMR: The biggest hidden downgrade

The roof, the roof, the roof is on fire

I’m starting off with the most important one, because I value your time. Also, attention spans aren’t what they used to be, and I want to get this one out before your eye catches something on Instagram.

We are talking about the single most important spec in modern hard drives. Which is also the one manufacturers try their hardest not to talk about: SMR or CMR.

Traditionally, CMR or Conventional Magnetic Recording drives write each track on the platter independently. This gives you a simple, consistent write performance.

SMR or Shingled Magnetic Recording overlaps data tracks like roof shingles. This is a great way to store more data in the same space, but it has the nasty side effect of requiring that adjacent tracks be rewritten when you write data to the disk.

During sustained writing, these types of drives can slow to an absolute crawl. This makes them a good choice for large file storage where you won’t be writing to disk often (e.g. a Plex server or incremental backup), but a poor choice for a NAS, where writing happens constantly with multiple users waiting to be served.

Hard drive technology is going beyond this and it’s important to keep up. HAMR drives or Heat-Assisted Magnetic Recording drives use a laser in the drive head to heat the platter to 450C, making it possible to write data more densely. Then there’s ePMR, which uses electric current to reduce timing jitter, letting the drive write bits more accurately and closely together, without the need for shingling.

The bottom line is, you need to know how a drive writes its bits onto its platters to understand whether it’s right for your needs.