My mac mini boots from an external SSD connected via USB 3. Didn't have any issue until i upgrade to Sierra. Most of the times i boot it load the white bar under Apple logo till 30% - 50% and after that it never boots in system i have to force shut down holding power button and boot again. Some time i need to do it 2 / 3 times before i can actually boot. I hear a disk spinning not sure if an external hard drive or internal hard drive. What could it be? Any one with same issue?