I'm interested in buying a brand new and sealed Late 2015 27" iMac with 2TB Fusion Drive from somebody. This model will have 128GB SSD directly mounted I believe, plus the HDD. My question is, can I use an external SSD via cable (USB or Thunderbolt) rather than the slow internal HDD? So I would have 128GB SSD on board and the external SSD to speed it up? Does this method work well?