Curly problem. I need to install Yosemite on an external drive, so I downloaded the installer, alas, the installer can't be opened by a Mac running Sierra, the installer is too old. Options: 1- wait til tomorrow when I have access to a Mac on Yosemite. 2- boot into a Yosemite recovery drive I have on a USB stick and reinstall from there, hoping it doesn't over-ride Sierra. I chose option 3- last week I created a startup USB stick on Yosemite consisting of TechTool Pro & 9 other apps. It uses 8.09 GB of space on a 16 GB stick, so there was enough room for the 5.72 GB installer as well.
So I booted the MBA into the Mac OS X Basics drive & ran the installer from that environment. Allegedly, the installation has "About 3 minutes temaining….."