Can't see Ext. HD with Sierra in Startup Disk options.

Hi there.

I think this is the model of my MacBook Pro, these are the specs I found - 15”, 2.3 Ghz processor, mid 2012 with Intel Core i7.

I’m no techo so it’s gibberish to me. I still have OS 10.8.5 on my internal hard drive because I still use Compressor and DVD Studio Pro, and have a few Garageband tracks I wish to keep.

However, I downloaded Yosemite 10.10.5 onto an external hard drive and have been booting up on that for the last 3 years. So I downloaded Sierra onto another external hard drive thinking I could do the same thing. (I want Sierra so I can update my Premier Pro version). It appears on my desktop as an external drive but when I go to Startup Disk, it doesn’t appear as an option.

Is there something technical that prevents me from doing this? (Yes, I might be a bit behind the times but will hopefully be up-grading at year’s end).

Any help will be appreciated.