I would like to install El Capitan on a Sandisk 128 GB Extreme Pro flash drive to use an alternate startup disk for my Mac Pro or MacBook Pro. I have found instructions online to make an El Capitan Installer out of the flash drive but the only thing you can do with it after that is to use it to install OS X on other drives. You can't startup and run from it like you would your primary startup drive.
I have the 6.2 GB "Install OS X El Capitan.app" and "InstallMacOSX.dmg" on a couple of the three HDDs I have in my Mac Pro, but I have not been able to figure out how to install a copy of El Capitan (which is the OS I am using) onto the Sandisk.
Any help would be appreciated.