Does the MacOsSierra app have to be on the partition I'm trying to install on? I ran it from my computer desktop and then chose to install it on the external HD partition.
If the installer offered you the partition as an option it should have completed the installation.
As I said, I've installed Sierra on a partition other than my boot volume. (It was on my internal drive, not my external, but that shouldn't matter.)
Dunno what's up here.
Hmmm... Did you look inside MacOS Install Data to see if there's a clickable option?
