You are right. However, the installer offers to first update the booted disk (green arrow) by default. One can only manually choose the other disk(s). Just updated Microsoft autoupdate and it did not even ask me where to go: updated the booted disk as well. Therefore, my impression that the installers do not install something "by mistake" and try the booted disk first still stands.

Alex
3.1 GHz 13" MacBook Pro 2015, 8 GB RAM, OS 10.11.2, Office 2011, TimeWarner Cable
2.8 GHz Xeon Mac Pro 2010, 16 GB RAM, OS 10.11.2, Office 2011, LAN