I have a similar problem. One of the main reasons that I purchased this phone is because it supports an sd card, but now it won't even take one. I have tried several different cards. The first card was a new Samsung EVO 32GB sd card. It worked great for about a month, then my phone began malfunctioning. It would only show a blank home screen and wouldn't function. After restarting and powering down my phone a few times, I was finally able to pull up my settings for a few seconds before it malfunctioned again. I had to keep restarting over and over before I was able to unmount my sd card. My phone stopped malfunctioning immediately after unmounting the sd card.
I thought that it was just a bad sd card, so I ordered a Transcend 64GB sd card. It only worked for two weeks before doing the same thing. I then used my old SanDisk 32GB sd card. It worked great for about a week, then my phone began unmounting then remounting the sd card many times a day. I tried taking the card out and spraying the card slot with compressed air, but it didn't work. That card began malfunctioning again, so I had to take it out. At least I was able to back up all my new sd card data to the cloud before it began malfunctioning again.
So now, I have no external memory storage and very limited internal storage space. I just bought this phone at Best Buy a little over two months ago, so I'm going to call Samsung to replace my phone. I am pissed.