What’s the model number of your drive?
See if this helps.
http://community.wd.com/t5/External-Drives-for-Mac/Disk-inserted-not-readable-by-this-computer/m-p/753790#M9718
http://community.wd.com/t5/External-Drives-for-Mac/partitions-and-slices-passport-for-mac-not-found/m-p/693942/highlight/true#M9364