I had the same problem when I first purchased SX OS. It was sd card related. Try it with a different card first. Is your switch updated enough to support eXfat? Try fat32 first then update then try exfat.
AFAIK the switch needs to be on a certain FW before it will recognize exfat. What I had to do is use a small card that was fat32 and get sx os all up and running. Then I had to do an 'offline' update to the switch to support exfat.