It’s the hardware and software, probably a physical problem with the SD card reader and Intel chipset issues combined with flaky drivers from Microsoft. I also used to be able to get a 64 GB card working intermittently but now only 16 GB cards and smaller work. Using the old Windows 8.1 drivers didn’t solve the problem, I got slow transfers and the card would get unmounted often.
I’ve given up and now I use a tiny ReMax USB OTG adapter coupled to a tiny MicroSD card reader: the combo is slightly larger than my thumbnail and I can use a 128 GB card with no issues. The card is only used for backups and movies so it’s not a problem to plug it into the USB port.