If you are having issues with your SDXC card, you may find the following article helpful, in particular this portion:
Can I reformat an SD card with Disk Utility?
Yes. Using Disk Utility, you can partition and format an SD device as FAT32 (using the MS-DOS FAT setting) or Mac OS Extended. The Mac OS Extended format can only be used on Macintosh systems. Cards formatted to Mac OS Extended will not be recognized by non-Apple systems. Formatting cards larger than 32GB for use with digital cameras, GPS, and other devices may require formatting them with the exFAT file system. When in doubt, format the cards in the device you intend to use with the card.