I had this happen twice. The first time, the card I had became unusable and I had to get another (and I blamed the card). By chance, is your card formatted as expandable (internal) storage? That is how I had mine set up originally. With my second card, I also set it up that way. When I had the message come up with the new card, I was freaking out that I would, yet again, lose all my data. I was able to get it to recognize the card be removing it and replacing it, and restarting the phone. I moved everything to the phone and reformatted it as removable storage. I don't know if that's the cause, since I am not the only one to use an SD card that way, but it is not something I'd prefer to have to deal with, especially on a flagship phone in 2016.