I has one of those grey and red Sandisk Ultra 64 GB it initially failed on my Note 2. I had about 40 GB on it and it quit being reliable then writing any more data, could not even erase it. Got an RMA broke the card in two pieces and sent it back to Sandisk. Got a new one. Rinse and Repeat for the Note 4 last week. I expect it back in a week or so and will reload my 40 GB of music, videos and books and see how it works then. Give Sandisk a call, it is warrantied for 10 years.
Backup, backup, backup...