If I now buy the current Passport for Mac I presume can simply plug into my MacBook Pro running on El Capitan version 10.11.6. Will there be any issues when I upgrade to Sierra?
It would be best to clarify the exact model number of the unit since there are different generations of the WD Passport for Mac. Assuming we are referring to the latest family of WD Passport for Mac units (WDBFKF00xxBBK), then rest assured the unit has been certified to be compatible.
However, system upgrade precautions should still be considered. Your unit should be completely disconnected during the entire upgrade progress.
Thanks for the information. Much appreciated