booting external FW800 snow leopard HD on brand new MBP?

Hi everyone,

I've never had such a seemingly unique problem; I normally boot my 21.5" iMac (Late 2011) off of a FireWire 800 external drive I have set up, and that boots up as snow leopard and works perfectly fine. Now, I moved to college and bought a Mid 2012, 15" quad core i7 MBP and when I connect the external drive to boot up off it, the computer gets stuck on the all grey apple symbol screen and will not boot. However, the HD is perfectly fine on the laptop's built in desktop, and I even verified/repaired it thru Disk Utility. I can get information off of it no problem, but it will not serve it's main purpose (booting).

Is this an issue with the current MBP? Are they incapable of booting lower operating systems? With an even better processor than before I don't understand why the laptop won't accept my HD but my iMac does.

Like I said, I have no idea why the laptop won't accept my HD to boot off; I was thinking maybe I could buy a firewire to thunderbolt converter and see if that would help, but I don't really have the money to just try that (unless that seems like a real plausible fix).

Thank you so much in advance.