MacBook Pro (Mid 12, 13") not booting OS X Yosemite - "No sign" on boot
Fixed it. It's so curious I didn't thought this was even possible. When I removed the connector of my optical drive, the macbook starts. When I connect it, it shows me the "no sign"

1Mio. Dollar Question: Why doesn't it boot with SSD but with HDD?

Edit:

The bug wasn't the SSD or the HDD, it was the the HDD flex cable. Firstly I connected my SSD with an OpticalDrive-to-2,5"-Drive-Adapter and it worked like a charm. So the only possible defect should be the flex cable. Bought one on ebay for 40 bucks and replaced it. Now the macbook works without any problems.

Edit #2:

Apple already claimed there is/was a fault with the HDD flex cable. But it's not public stated - so you had to ask apple. Found this news article – So I just called Apple, because I already replaced it on my own. Apple said that they can't replace it "again" but they can refund my replacement. So 2 weeks later I got a full refund of be ebay invoice.