Most of the questions here similar to my question regard triple booting and using rEFInd, which I'm not trying to do.
My Mac drive is only 128GB, and I've cut into that with Bootcamp, so I'm looking to boot Linux distros via microSD. So far, I've only gotten a Chrome OS on a microSD workable from the Mac's boot menu, but I'm failing to get Ubuntu.
This mentions something similar: Linux on Macbook, dual-boot, without rEFind, but it's for Dual boot. I did try taking their advice of copying the ../ubuntu/grubx64.efi to ../boot/bootx64.efi, but the results were goofy. With the microSD inserted, the boot menu still only displayed MacBook and Windows, and when selecting Windows, I got the MokManager error, but it still booted into Ubuntu anyway after leaving it for about 30 seconds. When removing the microSD and booting to that volume, Windows boots to the Grub bash screen. The only way I've found to fix this is to replace bootx64.efi with the Windows version. Basically the same problem you run into after installing Linux to an external drive when Bootcamp is already using that EFI folder.
When I installed Ubuntu to the microSD (/dev/sdc), I also installed the grub bootloader to the sdc volume which I was installing Ubuntu on, so that the bootloader did not damage my Mac OS or Windows boot processes.
I have tried deleting the Secure Boot option (shimx64.efi) to deal with this, but that causes Ubuntu to fail to boot anymore from microSD. So does replacing the Windows/bootmgfw.efi -> boot/bootx64.efi, although that lets me boot Windows again.
Of course, I'm trying to get it so that a separate menu item that says "Ubuntu" (and all future distros) when inserted, as I'd like to make use of other microSDs and USBs for the same purpose.
How should I go about altering the EFI folder and items, without causing Bootcamp to fail? (I've already mucked up that boot several times when testing different options, so I keep restoring from a backup folder.)
Is there a file in the EFI folder I need to edit to add volumes to be added to the boot menu?