I have a mysterious problem with my 2009 Mac Pro (upgraded to 5,1 12-core 3.33ghz) running 10.11.5 For some reason the machine will not boot without a certain, specific 500GB SSD in a bay. Recently I decided I wanted a bigger primary project-drive (I do audio production) so I copied all my current project files from the old 500GB SSD Evo840 project-drive to a new 1TB Evo850 via an eSATA dock. I shut down, removed the sled, residing in Bay 1 with the 500GB SSD and replaced it with the 1TB SSD. After replacing the sled the Mac Pro would not boot. I get a Chime and then nothing. If I put the old 500GB back it boots up no problem. If I put the new 1TB in another bay it boots np. But no matter what I do I can't get the MP to boot without that 500GB SSD in a bay. Also, if I move the mysterious 500GB SSD to any other bay than Bay 1 it takes almost two minutes longer for the Mac to boot. I've tried PRAM and SMC resets. My boot drive resides in the extra optical bay so I tried putting the enigmatic 500GB SSD in the optical bay and putting the boot drive in Bay 1. That config prevents booting as well. Everything runs fine as long as that particular drive is present in Bay 1. I trashed all the files on the 500GB SSD drive so it's just an empty volume now! I don't like that my entire system (and my career!) is beholden to that particular SSD. Any ideas on how I can be free of this phantom SSD? Thanks!