Been having the same issue with my 200GB SD card also from SanDisk on my 950XL, I'm on the latest build of the preview program, and after I factory reset my phone, my card stopped working. I'm currently trying to fix the problem. I'll let you know if I succeed.
Update: After*a few hours, a few downloads of formatting programs, putting it on different phones, and much patience I got it to work again with my Lumia 950XL. None of the*things mentioned above worked for me, what ended up doing the job was letting my 950XL run out of battery and turn off, after that I popped the SD card back in, and plugged my phone to get it charging, and VOILA! my SD card is back in business again.*I'm not sure why this was the method that worked, but I'm glad it worked again. My only concern now is letting my phone run out of juice or an unexpected reboot might make the card fail me again. Gonna have to be extra careful and not let my phone reset, reboot or run out of battery, which will be kind of annoying. Anyways, hope*my input helped. Have a great one!