WD external drive won't mount in OSX - partition map rebuild?
Seems like you need the following: Fixing my external hard drive&#39;s partition map

The fact that it's external shouldn't make a difference to you.

What happens when you try to mount it?

eg.

diskutil mount /dev/disk2s3



  alex@smiley:~|⇒  diskutil mount Usage:   diskutil mount [readOnly]
  [-mountPoint Path] DiskIdentifier|DeviceNode Mount will mount the
  volume in the "standard" place (/Volumes), unless an optional mount
  point is specified.


The error you get should point you to your problem.