External HD not booting anymore

Hi all

I just installed OSX Sierra on an external HD to try some old apps to see if it will work with it. After a few reboots I can't reboot no more and in the startup disk prefs it isn't there no more BUT the HD icon appears in the finder and I can get access to it too. So is there something that I'm doing wrong for this to happen? I didn't get any errors telling me that I didn't shut it off or something, I just want to know what I COULD of done wrong. Is there a way to fix this without reinstalling everything from the start?

THANKS!!!

Serge