I have a Mid 2012 Mac Pro with four operating systems on a mix of SSD and spin drives. High Sierra, Mojave, El Capitan and Yosemite and I want to be able to choose the OS it boots up into initially. It always boots up with El Capitan and in startup disk, the only option other than El Capitan is Yosemite. To see then others I have to hold the option key on start up and choose that way. It works, but if I reboot the machine again it defaults to El Capitan regardless of the OS I was using when I switched off. What I would like to do is be able to boot up with High Sierra at the outset. It's not a massive problem by any standards but I just wondered if there was a way as in High Sierra's startup disk options, I can see all four operating systems