"SD card is blank or has unsupported filesystems" message in Samsung Galaxy S4
SmellierMoans. Sometimes,your phone may not recognize your SD cards properly. Unfortunately, my phone is also one of them. While, each time it gives me the option to format my SD card, I tap "No" and then everything will be ok. All contents on my SD card can be shown normally. So, I think, you may choose not to consent the format. Just shut down  your S4 and power on it to have a try. Good luck.