External Hard Drive that won’t mount on OS X

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:
diskutil list

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.