Be aware that the SSD may consume more current than most USB ports are capable of providing and that would explain why the disk is not seen by the computer, or in my case, why the disk disconnects when in heavy usage. My 850 SSD states 1.4A peak current. My USB ports can only provide 900mA. It seems now I need to find out an external closure with two USB 3.0 connections, or maybe get a 3.1/USB-C enclusure and upgrade my Mac. Sorry, not a solution, just more details on the possible issue.