There is a little trick to what kind of memory cards the Nikon D850 takes. That’s because of a tweak that Nikon made with a firmware update a few years after the camera was released.
If you look at Nikon’s official tech specs for the D850 or the user manual, you’ll see that they list compatible storage media as XQD cards and SD cards.  That’s true, but it’s not the whole story.
There’s now also a third option that was enabled through a firmware update that Nikon issued quite recently (and rest assured that yes, this is an official firmware update from Nikon, not a third-party hacked version). Specifically, it was added in “C” firmware version 1.20, which was released in December 2020. (You can download the firmware from Nikon’s site, and I have a separate post that goes into detail on finding and updating the D850’s firmware.)
That firmware added a third type of memory card that’s compatible with the D850: CFexpress B.
So what’s the advantage of CFexpress B? It’s not just about broader compatibility. CFexpress B is basically a newer evolution of XQD. And in practical terms, it’s both much faster, and the cards are also becoming more widely available, leading to more choice and lower prices. 
Memory Cards for the Nikon D850
Number of Memory Card Slots. The D850 has two memory card slots, but they’re not both the same. One slot is for XQD/CFexpress B cards. The other is for SDXC/SDHC/SD cards.
Even though the D850 has two memory card slots, you don’t need to fill both of them for the camera to work. If you only have an SDXC card, for instance, you can go ahead and use the camera with just that.
There are, however, advantages to filling both memory card slots. The first advantage is that you’ll get several extra options, such as being able to save images to both simultaneously in order to have a redundant backup copy; being able to copy still images to one card and video to another; and treating them as a sequence of two cards, so that when it fills one, it just moves to the other, which gives you the maximum amount of storage space.
A second advantage–or, at least, an advantage to making sure that you’re using the XQD slot either alone or paired with the SD slot–is that CFexpress B and XQD cards have much faster transfer speeds than even the fastest SDXC cards. And that will benefit how quickly images will save from the camera’s buffer and ensure that high-bitrate video recording modes will breeze along (and, potentially, much faster downloads to your computer).
Memory Card Types
CFexpress B. With a firmware update in December 2020 (C:Ver.1.20), Nikon made the XQD slot also able to accept CFexpress B cards. 
It’s important to note the “B” here. Confusingly, there are currently three types of CFexpress cards, and they’re all different physical sizes (aka form factors), and they’re not interchangeable (that is, a CFexpress A card won’t work–or even fit–in a CFexpress B slot). 
CFexpress A is the same physical size as SD cards, and that’s the type used in some cameras, such as the Sony A1.
What you want for the Nikon D850 is the CFexpress B. You’ll sometimes see it written as CFexpress (Type B). CFexpress B cards are the same size and have the same contacts as an XQD card, which is what makes this improvement possible.
If you try using a CFexpress A or CFexpress C card in the D850, it won’t work. 
B&H Photo has an excellent selection of CFexpress B cards from manufacturers like Delkin, SanDisk, Lexar, Sony, ProGrade Digital, and Angelbird. You can find them here.
- Records 8K, 6K & 4K Video at High Frame Rates & Bitrates
- RAW Continuous-Burst Ready
- Read speeds of up to 1700MB/s and 1400MB/s write speeds** offer low latency during high-speed recording...
- Enables smooth, RAW 4K video(1) | (1)4K video (4069x2160p) support may vary based on host device, file...
- LIGHTING FAST WRITE SPEED – Capture 4K, 6K and beyond with our Write Speed of up to 1500MB/s. Great for...
- XQD USERS UPGRADE TO THE LATEST TECHNOLOGY: Backward compatible with XQD devices with firmware update...
XQD. XQD is an older format, and while these cards are quick, they’re not nearly as quick as CFexpress B cards. And since the price of CFexpress B cards has come down considerably, there’s not much practical advantage in choosing XQD over CFexpress B unless other parts of your workflow are geared towards XQD.
B&H has a good selection of XQD cards. You can find them here.
- Speed: The cards feature a maximum read speed of 440MB/s and maximum write speed of 400MB/s
- Compatibility: The Nikon 64GB XQD Memory Card is a high speed XQD format memory card for use with high...
SDXC/SDHC/SD. SD cards have been around much longer and are very widely available. They’re inexpensive, and you can get very large storage capacities with them. The downside is that even the fastest SDXC card is much slower than an XQD or CFexpress B card.
For the best performance in the D850, you’ll generally want to use a fast UHS-II SDXC card. And with the large images and high-bitrate video, larger storage capacities are much more practical.
I have some practical recommendations for fast SD cards here.
Things Worth Knowing About Nikon D850 Memory Cards
Camera not recognizing CFexpress B card. If your D850 isn’t recognizing your CFexpress B card, the most likely cause is that the camera’s firmware needs to be updated. Firmware version C:Ver.1.20 was released in December 2020, after the D850 had been on the market already for a few years.  And, unlike phones and some other devices, the D850 does not update its software automatically.
Memory Card Readers. Don’t forget that you’ll need a compatible card reader for downloading the images to your computer. A CFexpress B card will physically fit in an XQD card reader, but just as with the camera, the hardware needs to be compatible. There are some cross-compatible readers available, like this one, but you can’t assume your old XQD reader will work with CFexpress B cards.
I personally use Prograde Digital Readers these days and have had excellent results. They’re high-end readers, but I’ve found them to be very fast and reliable. They make an XQD/CFexpress B reader with a Thunderbolt 3 connection and CFexpress B readers, but not dedicated XQD-only readers.
- DO MORE OF WHAT YOU LOVE: Because the ProGrade Digital Thunderbolt 3 Single-Slot CFexpress Type-B & XQD...
- DESIGNED FOR LIFE ON THE ROAD: Wherever you go, your included adhesive metal plate attaches the magnetic...
- For XQD and CFexpress Type B cards
- USB 3.1 Gen 2 Interface
You can also find cheaper options from third-party manufacturers.
You don’t run into the same issues with SD readers–they’re very widely available. That said, speed and performance vary widely. If you’re using a UHS-II card, you get better performance from a UHS-II-compatible memory card reader. Prograde Digital makes some good ones, including a dual-slot card that has both CFexpress B and UHS-II SDXC slots (this is the one I use).
- DO MORE OF WHAT YOU LOVE: Because the ProGrade Digital™ USB 3.2 Gen 2 CFexpress™...
- DESIGNED FOR LIFE ON THE ROAD: Wherever you go, your included adhesive metal plate attaches the...
When you go to format the memory cards using the camera’s menu system, it will still refer to the XQD card even after the firmware upgrade and regardless of whether you’re using an XQD or CFexpress B card. (Tip: There’s also a shortcut method to formatting the memory cards.)
You have some control over managing how the memory cards in the D850 are used. You can treat them as sequential memory so that when one card fills up it automatically switches over to the next one. You can copy to both simultaneously to create a mirrored backup version. Or you can send image files to one and video files to another. I have a more detailed post on that here.
And there’s a shortcut to format memory cards in the D850. You can find more information on that here.
You can also copy from one memory card to another in the D850. I have a post on how to do that here.
There’s also a CFast memory card format. That’s different again. It’s an older format form factor that was an evolution from CompactFlash (CF). It’s not compatible with CFexpress.
- More specifically, it lists: 1 XQD card and 1 Secure Digital (SD) card; SD; SDHC (UHS-II compliant); SDXC (UHS-II compliant); XQD Type Memory.
- At the time of writing, most of the currently available XQD cards are rated for speeds of around 440 MB/s read and 400 MB/s write. Most of the currently available CFexpress B cards are rated for around 1700 MB/s read and at least 1200 MB/s write (or faster).
- A year earlier, in December 2019, Nikon had issued firmware earlier for the Z6 and Z7 that had added CFexpress B support to those cameras.
- Even more confusingly, there are also versions within each of these form factors. Those affect compatibility but aren’t related to the physical size of the cards.
- While CFexpress A and SD cards have the same form factor, and you can physically fit a CFexpress A card into the D850’s SD slot, the D850 does not have compatibility with CFexpress A using the SD slot.
- The D850 hit the market in mid- to late-2017.
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