NO, it's not.

I had S3's before I upgraded to the S5, and went through three of them, and countless SD cards. Seemed the S3 "ate" SD cards, as in all (but one) of the cases, working cards suddenly stopped working -- like yours.

When I switched to a Samsung SD card, in the last S3, the problem stopped.

And, it has not happened since with my S5.

What I learned was to copy anything I needed to keep OFF the SD card onto my PC as soon as possible.

My suggestion would have been to switch to a Samsung SD card, but since you've already tried that, don't know what to suggest.

 

I am using a sandisk SD card in my Verizon S5 and not having that problem. Mary

 

Save the info on the PC. Format the SD card with the PC.

Put it back in the phone, and mount it if necessary ( mine did not need too )

Now, format the SD card again with the phone.

By now, the phone can see the SD card and make use of it.

If none of the above work, replace it.