WD external drive won't mount in OSX - partition map rebuild?
Solution 1:
Seems like you need the following: Fixing my external hard drive'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.