Okay it seems like some models of MacBook Pro have issues with a fast (6Gbit/s) SSD in the optical bay. Even though my Mac says it offers 6Gbit on both bays it simply didn't work in the optical bay. So I switched places between the two drives and everything seems to work. My Mac booted from the old HDD in optical bay automatically and then I cloned it afterwards and changed the startup disk to the new one. As said before, don't put a fast SSD in the optical bay!