!! Hard drive !! The Harddrive with the OS X on it for booting, has to be to be in the Harddisk bay and not the optical bay!
The System does only startup regularly on the SATA connector meant for the Harddrive, not the SATA connector of the former optical drive! (Even for MID 2012)
So, cause the most of us will add a SSD and want to run the System from it, we have to change the old Harddisk to the Optical bay where it is mounted without the anti shock rubbers.
In my case it did´t die there for over a year yet, so, not the big deal it seems at the first look ;-) lucky