there are multiple posts on here about the card being bad or the reader being bad. I know if I have a 32 Sandisk Ultra and it starts having mount/unmount issues and then it fails to be recognized on my phone or macbook, then it is most likely the card. If I then put a 16gb card into the same phone and same computer that I just attempted to use the 32 gb card on, and the 16 gb card works, then it isn't the phone or phone reader or computer or computer reader.
Some of these 32 gb Sandisk Ultra cards are pure s h it ...no other way to put it. I am using one right now, again, after having my 32gb crap out on me. So far, so could with a few ROM flashes as well. I am not sure if they are designed for crap or I got a decent card on my second try. If I ever have another problem with my 32 gb Sandisk Ultra then I will switch brands for sure.
as I have posted before. I think there may be an aspect of stray electronic voltage from electrostatic discharge happening when handling the card. It is such a small card that it is almost impossible to avoid touching the contacts. The larger sd card is much easier to handle and avoid this issue.. I am very careful now with my handling procedures.