Trouble backing up my Mac (with Time Machine) to my WD passport

You can also use the disk utility app to test the passport, try that and see what happens, if the test completes successfully the problem could be with time machine or some other software that is causing the conflict. Check the link below for the steps.

http://wdc.custhelp.com/app/answers/detail/a_id/866/session/L3RpbWUvMTM0OTIwODA0OS9zaWQvSWFoWjhLN2w%3D