I came here with the same problem. I bought two 64Gb Sandisk MicroSDXC cards with adapters. Firstly, the fit of the adapters is horrible, which is probably a big reason for the failure rate. I don’t have any devices that use the microchip directly, so I had no choice. It shouldn’t matter.
Neither of my Nikon cameras, a D5100 and a Coolpix S32 could read or format the card. My Mac Pro desktop could see the chip, but couldn’t format or write to it. It reported read only mode. I even tried an external 3rd part SDXC reader to see if that would make a difference. Nope.
I took the cards to work and tried on my 64bit Win7 machine. Nope, same thing. Tried on my iMac and got same problem.
Came here, and downloaded the SDFormatter - same problem - chips are read only. Couldn’t do a thing with them. I almost gave up. But then I remembered that my Macbook Pro laptop (from 2011) had an SD card reader built in. Can’t recall ever using it. Popped a card in a voila - it read the card and allowed me to format using the Apple Disk Utility.
After that, both cards now can be read and written to by both cameras. But, neither camera can format the cards.
It’s really weird. I don’t know that I can trust them.