Boot from different startup disk AND in safe or recovery mode I have two versions of Mac OS on my mac pro: Mojave, and 10.8.5.

While using Mojave, I selected the startup disk to the other one via Preference Pane in order to access some old programs. 

To switch back to Mojave, I tried the same thing, but 10.8 doesn’t recognise the volume where Mojave is (I imagine due to the new file system), so the option doesn’t even show up in 10.8’s Startup Disk preferences.

I thought - no problems, I’ll do it while booting, by pressing OPTION. The startup disk selection screen appears correctly. However, after I select Mojave this way, the loading screen gets stuck.

So what I would need to do is  (1) press OPTION during startup to select Mojave as the boot disk, AND (2) launch safe (or recovery mode) while that one is booting. However I haven’t figured out a way to do this. Any ideas?

Thanks!
The rEFInd Boot Manager will allow you to select to boot from a recovery volume. 

Use the following steps to install rEFInd.


Boot back to OS X 10.8.5.
Download  rEFInd.
Assuming you have downloaded rEFInd to your Downloads folder,  you can install rEFInd by entering the following commands in a Terminal application window.

cd ~/Downloads/refind-bin-0.11.4
sudo ./refind-install

Restart your Mac.
From options shown on the rEFInd menu, select to boot from a recover volume.


If you change the default so the Mac no longer boots to rEFInd, then following commands can be entered from OS X 10.8.5 to change the default back to rEFInd.

cd ~
mkdir efi
sudo mount -t msdos /dev/disk0s1 efi
sudo bless --mount efi --setBoot --file efi/EFI/refind/refind_x64.efi --shortform
sudo umount efi
rmdir efi


rEFInd can be removed by using the following commands.

cd ~
mkdir efi
sudo mount -t msdos /dev/disk0s1 efi
cd efi/EFI
rm -r refind tools
cd ~
sudo umount efi
rmdir efi