I had a SanDisk SDHC 32gig go bad on me.
At first I could transfer files from the PC to it using a USB adapter, then it would read as being full. Formatting did no good, as it did the same thing.
Finally, the phone refused to read it as being in the phone. I contacted SanDisk about it and got an RMA. They sent me a new one yesterday.
I did mention to them that there are a lot of people having their phones trash the cards.
Go to the SanDisk website, create an account, and register your cards. It will help with replacements.
In the meantime, I bought a SanDisk SDXC 64gig card for the phone. It's formatted exFAT from the factory. Before installing it in the phone, I turned it off and removed the battery.
Put it in, replaced the battery, and turned it on. The phone read it correctly, and I'm (so far) good to go.