Macpro two drives Triple UEFI Boot move from Grub2 to "headless" bootmanager rEFInd issues
The original assumption was rEFInd would be installed in a small FAT formatted partition. This had the advantage that macOS, Windows and Linux could read and write to the volume. Recently, I added the option for rEFInd to reside in an EFI partition. This required modifying the Startup application to include an option for mounting and unmounting volumes.

One can define menuentry options in the refind.conf file. The default menuentry to boot can be selected by using the default_selection option. The refind.conf file can also contain a include option to read options from another file. For example, the option given below would read options from the default_os.conf file in the rEFInd tools folder.

include \EFI\tools\default_os.conf


The Startup application assumes the default_os.conf file contains default_selection options where zero to one option is not comment out. For example, the file could contain the following lines.

#default_selection "macOS High Sierra"
default_selection "Windows 10"
#default_selection "Ubuntu 18"
#default_selection "High Sierra on USB3"
#xdefault_selection "Fedora 18"


Here "Windows 10" would be the operating system that rEFInd would boot by default. The Startup application can change which default_selection options are commented out.

If you were to select High Sierra on USB3 as shown below, the the default_os.conf file  would change to what is shown below.



#default_selection "macOS High Sierra"
#default_selection "Windows 10"
#default_selection "Ubuntu 18"
default_selection "High Sierra on USB3"
#xdefault_selection "Fedora 18"


If you were to select Use Previous as shown below, then the default_os.conf file  would change to what is shown below. Basically, rEFInd would now boot to which ever operating system was chosen last from rEFInd's menu. In your case, you probably would configure the Startup application to omit this choice.



#default_selection "macOS High Sierra"
#default_selection "Windows 10"
#default_selection "Ubuntu 18"
#default_selection "High Sierra on USB3"
#xdefault_selection "Fedora 18"


Startup Application User Configurable Options

Being that you have the source, you can actually make any changes you like. However, the top of the script contains a few common items one would wish to configure.


Path an name of the file containing the default_selection options.

set inputFile to "/Volumes/REFIND/EFI/tools/default_os.conf"

An optional device to mount. If you do not need to mount any volumes, then leave the string empty. For example, to mount the default EFI partition, you would make the following change.

set deviceToMount to "/dev/disk0s1"

If you need to enter a password before mounting a volume, then make the following change.

set mountAsSuperuser to true

To omit Use Previous from the list of choices, you would need to make the change shown below.

set allowPrevious to false



Downloading the Startup Application

The Startup Application can be obtained by downloading the Startup.app.zip file. Basically, click on the green Download button on this webpage. This application was creating using macOS 10.13.16 (High Sierra) and the Script Editor application Version 2.10 (194) for AppleScript 2.7. You will need to enter the following command to remove the quarantine. Here I assume you download to your Downloads folder. 

xattr -d com.apple.quarantine ~/Downloads/Startup.app


If you are using an older version of macOS, then you may need to build the application. The steps are outlined in the next section.

Steps to Build the Startup Application


Download Startup.applescript and rEFInd.icns from this SourceForge website.
Open Startup.applescript in Script Editor application.
From the Script Editor menu bar select File-&gt;New.
Copy the entire contents of Startup.applescript to the new window.
From the Script Editor menu bar, select File-&gt;Save. Choose the Application file format, then set save as to Startup.app.
From the Finder application, right click on Startup.app and select Get Info.
Drag the icon representing rEFInd.icns onto the icon in the upper left corner of the Startup.app Info window, as shown below.

 
The result should be the Startup.app icon be changed to the image shown below.




Finally Note

There also is a GUI PowerShell version for Windows and a GUI Python 3 version for Ubuntu (and perhaps other versions of Linux). A CLI version written in Bash 3 exists for operating systems with no graphical interface.