I purchased a new SDXC 64GB card from Amazon
"SanDisk 64GB Mobile Ultra MicroSDXC Class 10 UHS-1 30MB/s Memory Card with SD adapter (NEW VERSION)"
I plugged the card into the microSD slot on my Surface Pro. All went well for a while. I set up the card to use the Windows 8 File History support and I copied about 5GB of music to the card. All this with no problems.
But, a couple of hours later I noticed that the card was gone from the list of drives (under Computer) in File Explorer. I could not access the SDXC card. I looked in the Event Log and there was a disk error event with event ID=11 and the message text:
"The driver detected a controller error on \Device\Harddisk1\DRxx" where the xx has varied (increasing with each error)"
I simply removed the SDXC card from the slot and reinserted it. It immediately showed up again in File Explorer. Since then the failure has occurred in a similar fashion about 10 times. The SDXC card seldom is in the slot for more than 1-2 hours before the failure occurs (drop from File Explorer, unable to access the card). There always is a disk error event similar to the one above associated with the failure. Each time I simply remove the card and reinsert it. Then I can run a few hours before it fails again.
I have a USB adapter which accepts the SDXC card. I have inserted the 64 GB SDXC card into the adapter and plugged it into the USB 3.0 port. I have run the card this way for over 24 hours and have never seen a failure while the SDXC is plugged in through the USB port (but who wants to use a Surface Pro with a USB dongle hanging out). Nevertheless, this does demonstrate that the failure is probably not associated with the SDXC card itself.
I have an 8GB SDHC card which I had been using in the Surface microSD slot. I ran several days with this without any problems before I purchased the 64GB SDXC card.
Just so people know, I am familiar with micro SDXC cards. I am certain that this is not a problem associated with inserting the SDXC card into the Surface slot.