This happened to me recently... I may have unplugged an external 5Tb harddrive without ejecting it from mac os x 13, and it could not be mounted again. I plugged the harddrive into a windows machine, run "repair" there (whatever that means) and the data was perfectly readable. It's just that the disk would not mount on mac.

Stackoverflow to the rescue! The solution is:

sudo pkill -f fsck

Or actually

ps aux | grep fsck 

sudo kill -9 <pid of the fsck process>

Did the trick for me. Then to make the hard drive writable, you have to run the Disk Utility "S.O.S.", unmount then mount it again.