Tried booting up SD + SSD (SSD plugged into pi 3). It appears the unit doesn't supply enough current to access the SSD correctly (get I/O errors, and fails to boot). I set this up this SD + SSD using a Motorola Lapdock which supplied enough power to the pi3 to boot up and run off SSD.
I find it odd a 18V 3A adapter doesn't supply enough power when a lapdock 19V 1.5A has no issues.
Tried again today and it worked. Looking good. will report again after a few days of use.