I have a sporadically unavailable 64GB SanDisk SDXC MicrosSD. Generally, when it becomes unavailable, that is, when the phone is unable to "see" the card or its contents, restarting the phone again makes it available. But sooner or later it again becomes unavailable.
I tried wiping the card with a microfiber cloth. When I reinserted it in my phone, it wasn't seen. Restarting the phone made it available.
My phone is a Galaxy S3 running Stockorama version2 with 4.1.2. The same problem existed, however, on my previous ROM.
My experience with the 64gb cards, is i have to format the card on my computer, my phone wont even see it to format it. I have a mac, and formatted it in MS DOS (FAT-32 i believe)
Hope it helps