SD Express Cards & microSD Express Cards Explained

As if the world of memory cards wasn’t confusing enough already, there are some new, emerging formats in the pipeline: SD Express and microSD Express.

As you can probably guess from their names, they’re evolutions of SD and microSD. And it sounds like they’re faster.

Which they are, and by a lot. But it’s not as simple as just plopping an “Express” version in your existing camera and getting blazing-fast new speeds.

What is SD Express (and microSD Express)?

SD Express was launched by the SD Association in 2019. The SD Association is the organization that develops and governs the specs for the SD and microSD memory card formats. 

The idea of SD Express and microSD Express is to keep the existing form factors but bring massive speed increases to keep pace with the demands of 8K and 3D video. (There’s a parallel effort focused on massive increases in storage space, known as SDUC/microSDUC, which is related but not directly the same thing.)

They do this by harnessing the speed of PCIe and NVMe interfaces. And the gains are impressive. The SD 8.0 spec brought the full-size SD Express cards up to nearly 4 GB/s. With the latest SD 9.1 spec, released in October 2023, it brings the smaller microSD Express up to nearly 2 GB/s speeds (1,969 MB/s, if you want to be pedantic). For comparison, the current generation of fastest SD cards max out at around 300 MB/s. But with that speed boost, it brings them up into the much faster speeds of memory card types like CFexpress. 

What’s New in the SD 9.1 Specification?

In October 2023, the SD Association released the SD 9.1 specification for SD Express.

This latest update integrates multi-stream access and refines power and thermal management. The multi-stream aspect, in particular, allows up to eight separate streams. The practical implication for photographers and videographers is that it can handle large files simultaneously while ensuring data integrity.  

• Multi-stream Access: This feature ensures that the minimum defined performance of the PCINVMe interface in SD Express cards is maintained, thereby enabling the simultaneous handling of up to eight different streams of data.
• Power and Thermal Management: The SD 9.1 specification addresses power consumption, designed to make data-intensive, high-bitrate recording more efficient and resulting in fewer overheating problems. 

SD Express Speed Class Categories

A related innovation with the release of SD 9.1 was the release of new SD Express Speed Class categories. The SD Association has a long history of developing speed class rating systems that are intended to make it easier for the consumer to choose the right card for their device. 

Unfortunately, though, they keep coming up with new systems, which has a tendency to cloud any clarity the ratings should provide. 

And they’ve done it again. Rather than sticking with the existing Video Speed Class rating system currently used on SD and microSD cards (the V30, V60, V90 ratings), they’ve come up with something exclusively for SD Express and microSD Express. It comes with its own symbol with an “E” followed by a data transfer rate (the “E” looks to me more like a British Pound symbol, but whatever).

SD Express Speed Class	Minimum Read/Write Performance
SD Express Speed Class 150	150 MB/s
SD Express Speed Class 300	300 MB/s
SD Express Speed Class 450	450 MB/s
SD Express Speed Class 600	600 MB/s

Importantly, these don’t refer to maximum peak transfer rates but rather to minimum sustained write speeds (aka minimum sequential performance levels). That’s specifically designed to be the useful measure for the constant stream of data that comes when recording video. 

And you’ll also notice that the current rating system maxes out well below the theoretical maximum speeds of the spec. In real-world use, it’s going to take quite some time before the actual cards catch up to the theoretical limits of the spec, if they ever do (hint: they rarely do before something shiny and new comes out to replace it).

These speed rating categories aren’t quite as simple as they first appear, though. That’s where the magic of multi-stream access comes into play. It means that bandwidth is allocated across the streams. For example, a card rated at SD Express Speed Class 600 can support 8 streams at 75MB/s or a single stream at 530 MB/s along with 7 other streams at 10 MB/s, and so on. 

SD Express & microSD Express Compatibility 

So this is where the catch is. Fast cards = good. But only if your device supports them. And that’s the stumbling point for now. 

The SD Express and microSD Express cards use the same form factors as existing SD and microSD cards. So they’re the same size and shape and fit in the same memory card slots. 

They’re also designed to be backward compatible. This means that an SD Express card will work in an existing camera that uses SD cards. 

But the catch is that you won’t get SD Express speeds unless the camera or device fully supports SD Express. More technically, both the card and host device have to be using the PCIe bus and both have to conform to the SD Express Speed Class access rules as they are defined in the specification. Otherwise, putting an SD Express card in a camera or host device that only supports regular old SD will result in regular old SD card speeds. 

You can already find SD Express cards for sale. But while it might seem like you’re getting a new, faster, better SD card, there’s no point paying the premium prices to buy one for the current generation of cameras.