WD Passport 1TB USB 3.0 Time Machine external HD not being recognized on Yosemite
Reboot your computer.

Login to your desktop

open up activity monitor.

Check to see if fsck is running
if fsck is running...


Your computer is attempting to repair the drive, so that is WHY it is not showing up in any of the places you are looking beacuse fsck has already unmounted it.
You have two options (both are risking or dealing with data loss so proceed at your own risk!! )
Option a: You can force quit fsck from activity monitor - then OSX should mount your drive with an error that looks like this...

Next steps for option a:
a1. you can attempt to backup and data that you do not have a copy of. Keep in mind the finder might hate copying large files out
a2. If the finder cannot copy files off, you can attempt to repair your file structure or &quot;preview&quot; your drive with disk warrior. To help you copy your files off.
Then assuming option a fails / succeeds and you do not need any data.
Option b:
version 1: If the drive is business critical, then replace it.
version 2: If this drive is for home use and not a huge deal if might die on you randomly then your best bet is to reformat your entire drive and then keeping a eye on it.