If you’re having problems with a WD Passport external hard drive for Mac failing to mount, or not being recognised or not being visible in Finder, it may be that you’re simply using too long a USB cable. The drive only functions properly when it is used with a short USB cable. I have had persistent problems with my WD passport drive, problems which have now been resolved by switching to a short 6-inch (12 centimetre) USB cable.
If you have had such problems, you may need to repair the volumes on your WD Passport drive using Disk Utility on the Mac. Highlight the external drive, select the First Aid tab, then click the Repair Disk option.