UHS-I vs UHS-II on microSD & SD Cards

Wondering what the difference is between UHS-I and UHS-II with SD and microSD cards? Here's an explanation.
SD Cards - UHS-I vs UHS-II
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If you’ve looked at buying an SD or microSD card lately, you might have run into some new codes: specifically, UHS-II.

It refers to something called ultra high-speed bus, which in turn refers to the interface between the card and the device. So far, there is UHS-I and UHS-II. There is a specification of UHS-III, but I’ve not seen any cards actually turn up using that yet.

UHS-II is the newer, and potentially faster, system but adoption is still not widespread. And a UHS-II isn’t necessarily guaranteed to be faster than every UHS-I card in practice. I’ve put quite a through microSD and SD cards through speed tests, and the UHS-II cards aren’t necessarily faster, at least for now.

Compatibility

The UHS-I and UHS-II specifications are designed to be backward compatible. That means that you can use a UHS-II card in a device that’s compatible with UHS-I and vice versa. But if you do so, you’ll be limited to the lower spec, in this case UHS-I. That is the general intention, at least. But it gets a little more complicated, because there are different implementations of UHS-I that have different speeds. Some cards fall back to a slower UHS-I spec; better cards fall back gracefully to a UHS-I spec known as SDR104. Unfortunately, that particular part of the card’s capabilities is rarely specified in either the packaging or the technical specs.

Lexar Professional 2000x 64GB SDXC UHS-II Card (LSD64GCBNA2000R)
  • High-speed performance leverages UHS-II technology (U3) for a read transfer speed up to 2000x (300MB/s)
  • Captures high-quality images and extended lengths of stunning 1080p full-HD, 3D, and 4K video with a DSLR...
But in many cases, it frankly doesn’t matter which you get. But that’s simply because the cameras and devices that can take full advantage of UHS-II are only just starting to become available.

And some that are UHS-II compatible can be a bit quirky. For instance, the FujiFilm X-Pro 2 camera has two SD slots. But only one uses UHS-II. So if you put a UHS-II SD card in the slot that has the UHS-I interface, you’ll be limited to UHS-I speeds.

But if you have a device that does use the UHS-II interface–and the technical specs or instruction manual will say so if it does–then there’s a good chance you’ll get better performance out of a UHS-II card.

One common device where this can become an issue is with memory card readers. Most of the card readers currently available support UHS-I cards. UHS-II cards will work, but they’ll be limited to UHS-I speeds. There are some card readers that do support UHS-II, such as the Lexar SR2 reader or the SanDisk Extreme Pro reader, but you can’t assume that every card reader does.

How to Tell the Difference Between UHS-I and UHS-II

There are two ways to tell the difference. The first is obvious: look at the label or packaging. They’ll have either UHS-I or UHS-II, or sometimes just I or II. Technically, it should be Roman numerals, but you’ll sometimes see it incorrectly written as a number 1, like UHS-1.

You can also tell them apart by looking at the cards themselves. UHS-I cards have a single row of pins on the back. UHS-II have two rows. In this example, both of these are SanDisk Extreme Pro cards, just different generations.

SD Cards - UHS-I vs UHS-II
At left, with the single row of contacts, is the UHS-I card. At right is the UHS-II card.

U1 vs U3 vs V30 vs V60 vs V90

The UHS-II code isn’t the only new code you’ll see on some microSD and SD cards. There’s also a new code that relates to suitability for recording high-resolution video. U1 and U3 have been around for a while; the new addition is V30.

All of these are speed classes aimed at recording high-resolution video. And they’re a separate designation from the UHS-I vs UHS-II one. I have more information on microSD speed classes here.

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