Stop sign at boot - new SSD I've got a mid 2012 MBP 13" in which I inserted a brand new MX100 in the place of the old HDD. I've put the old HDD in a case in the superdrive place and tried to install 10.10 but the install failed. 

I then tried with th SSD in an USB case, managed to install &amp; boot 10.10. Then I put the SSD back in the Mac and got the stop sign at boot. 

At first I thought it was the Firevault on the HDD which was causing the issue but even after turning it off, the mac didn't want to boot...

Which brings me 2 questions:


Why does it boot when plugged in an USB case and not from inside?
How can I fix this thing?
To solve the problems we have to place the HDD on the original place. Then we place the SSD to the SuperDrive place. 
Just install Yosemite as usual and everything is working now.