OK, this morning my brain has gone silly. I have a late 2015 27 iMac which came with an internal 1TB drive. I have cloned this installation to an external 120GB Samsung SSD and connected it via USB and successfully booted from it. Yes, I can tell a difference in boot times and the SSD+USB3 is faster. Anyway, how can one tell in 10.11.x which drive they are booted off of once they are already logged into the environment? I know how to do this in Windows, but I'm without brain this morning. I am trying to make sure the external SSD boots as a default. Any enlightenment is appreciated.