Here are some answers to the questions:
I assume then that it both OSes reside on the same drive. Is that correct?
SDA is El Capitan, SDB is Mint, SDC is DATA (A, B, C are separate drives)
Which Linux Mint? 17.3? 18.0? 18.1? 17.2? 17.1?
18.0
Did you have LM 17.3 and updated to 18.0 or 18.1...