I had the same problem with a SanDisk 64GB Ultra (class 10) card. Went through several rounds with SanDisk to have them finally say we will send you a mailing label and you send the card back and we'll replace it. I never received a RMA number or mailing label. Contacted them again and got the "Oh, we're sorry..." story and three weeks later I still have no avenue of return. Anyway...

I was having to reboot/disassemble/eject and repeat several times a day. I went out and bought a Samsung 64GB Pro (also class 10) from Best Buy on sale for $49.99. Placed it in the phone and formatted it, popped it out and transferred all my stuff onto it and haven't had the first problem since. It also seems to work better than the SanDisk (when it worked.) I never get a "hicup" now when watching a video or listening to music. I used to get those with the SanDisk SD. All that being said, go buy a Samsung card and enjoy your phone.

 

I bought a Samsung 64gb from amazon never had a problem still early days like had it about 3 month
norman

 

I had a Sandisc 64gb class 10 bought at Amazon w/ many issues. Returned it for a Samsung 32gb which I've not had any issues with.

Edit: The Samsung was class 10 too. I don't know why I didn't write that originally.