A few months ago, when I got my D850, I also invested in an XQD card besides the SD card so I could compare the two and have both the slots occupied in case of overflows.
The camera was still on older firmware and to update it, I would have to copy the firmware to the primary card which happened to be my XQD card.
The solution to this one was simple, I copied the firmware update to my SD card and simply made my SD card as the primary card.
Once the update was done, I switched the XQD card back to being the primary.
More recently, I ran into an issue which happens at times even with my earlier Nikon bodies…
For some strange reason, Lightroom refused to import more than a few files. The only way out, that I already knew from my earlier experience was to copy the files from the card and then import to Lightroom.
Since I never bothered to get an XQD card reader, this was an issue. Fortunately, the D850 offers a very simple card-to-card copy operation.
Once again, the solution was simple. I just copied the contents of my primary XQD card to the SD card and copied the files from the SD card to my computer.
Hope this small tip helps some people who have similar issues.