Hello, All. I have a relatively new D750 and attempted to insert a new SanDisk SD card into Slot #1. The camera would not accept it. I then removed the existing card from Slot #2 and the camera would not accept it into Slot #1 either. The new card can be inserted into Slot #2 with no issues. It seems as though the problem is with the camera and not the card. Any suggestions that I can try out?