The 64GB SDXC cards I've been using are Sandisk branded, and I got them from B&H, so they're probably genuine. But neither the built-in card reader in my Windows 8 laptop, or the Vivitar USB card reader I've got plugged into my Windows 10 desktop PC at home, seem to be able to write to the cards.
I guess it's possible that both readers are just not able to handle SDXC cards. I think I got the laptop in 2014. I got the desktop in 2017, and bought the USB card reader in an office supply store last week (but it could have been manufactured a while ago).
I'm probably going to get a couple of 32GB SDHC cards as soon as it's convenient, and see how those work.