Booting from second drive - forbidden sign

Hello, I currently have two SSDs, with two copies of OS X, I use one for work and one for the rest (I like to keep the apps and the files separated). My problem is that, while I can boot from the second SSD when it's connected externally, when I try to put it inside the macbook (i replaced the optical bay with an adapter for a second disk), the system sees it but refuses to boot from it, giving me the 'forbidden' sign. After a few seconds it then automatically boots form the ssd in the main hard disk bay. Why does that happen? Is it possible to have two boot disks both inside the macbook? It's annoying to carry around an external ssl and I'd like to put it inside and forget about it

My macbook pro is a 13'' mid 2012

Thanks in advance!