I’ve just purchased two WD Passport Mac portable external drives. Both perform normally in all respects, and pass all diagnostics. However, when I attempted to use the Mac firmware update software (downloaded from here), it didn’t recognize either drive. There was nothing else connected to the Mac’s USB ports. I’m running OS X 10.9.4 (which is the most current production OS X version).
I was able to work around this by connecting the drives to a PC and using the Windows version of the firmware update software (downloaded from here). It immediately recognized the drives and updated their firmware (from v1.007 to v1.012).
It appears to me that there is a problem with the Mac firmware update software that needs to be fixed.
I opened a ticket for this with WD Support, and received an essentially nonsensical response.