You can try to mount it from Terminal (in Utilities) by using the command diskutil list
to get its path and then
diskutil mount /dev/xxxxx.
Apparently there is a 4gb file size limit for MS DOS Fat on Mac. Find out more here. http://osxdaily.com/2012/04/22/format-drive-mac-pc-compatible/
You may want to back up the photos from the card and reformat it on your Mac as exFat.
(edited)