I had an external drive (Lacie) that had was partitioned for both Mac OS X and Windows that failed to mount on OS X. I was able to remedy this frustration and recover my data in the following way:
Target the disk that needs to be fixed:
In my case it is disk2s2.
Then run this file system check for exFAT:
sudo fsck_exfat -d disk2s2
It will then ask you: Main boot region needs to be updated. Update?
. . . . and you should be able to access the drive again, as it should mount.