If I knew that I would be smiling ;-)
The day after New Years Eve, I hit Update (to 10.11.3) The Mac did the update and as it wanted to boot... It would NEVER get past the boot-bar on the black screen. I tried for two days, using ALL knowledge I have gathered over 20 years with Mac, read Google ETC ETC ETC... The screen stayed black.
I had to backup and init the disk... And during re-backup given the fact that ANYONE using USB3 with a Retina MacBook Pro is living on a VERY dangerous edge where the OS is likely to just throw-out the USB3 drive WHENEVER it so pleases and in such a case EVEN destroy the Partion ---- OS X DID destroy my Volume. More precisely, it destroyed the EFI Partition. Anyway, I WAS able to (wasting 5 days) reconstruct ALL my data with the wonderful tool called Disk Drill.
So DUE to the 10.11.3 " UPDATE " I LOST about 2 weeks of work.
In the wake of this, I dumped ALL my USB3 drive and swapped them for Thunderbolt which I in my experience has NEVER exhibited SUCH issues.
I LIKE El Capitan and the idea of it. It IS a truly amazing OS. Only it is FAR from stable. If only Apple would fix what is IN the bug-reports, the OS would come back and mirror stability which I as OS X user (Until Mavericks) has gotten accustomed to... But by the release of a major upgrade every year, Apple is doing nothing but making their developers AND users Angry and FORCING them to STOP installing their beta software. Which is VERY BAD for bugs !!!