MP 1,1-5,1 Choose Startup OS/disk (not what you think)

[Wingston]

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

 

[DeltaMac]

You have the answer in your post. Use the Option bootpicker method. That's probably your only "option" for to select the system you want to boot, each time you boot.
I think this is because of the range of boot systems you have, and that the two oldest systems (Yosemite and El Capitan) do not understand what an APFS formatted boot drive is, and ignores the boot system on those drives (High Sierra and Mojave). Then, when you DO boot to either High Sierra or Mojave, you may be able to choose any of the boot systems from the Startup Disk--BUT, maybe the firmware does not allow the hardware to see the two newer systems, despite the fact that you might have one of those newer systems set in Startup Disk pref pane.
But, choosing with Option at each boot should let you choose from any drive with a supported system.

 

[reader50]

Boot into High Sierra using the Option key method.
Go to System Preferences -> Startup Disk
Select your High Sierra volume.
Close System Preferences.

From now on, booting should default to High Sierra.