is there a way to disable a drive bay from the command line? Regarding a previous question I asked a while ago:
Macpro 5,1 mid 2010 cannot boot to Mojave from Mountain Lion?
I am wondering, is there any sort of default writes or other software solution which would allow me to completely disable a drive bay? If I have both the Mojave and Mountain Lion drives loaded into the drive bays, the computer will always choose to boot Mountain Lion, and the only way I can get Mojave to boot is to physically pull out the Mountain Lion drive.  I am just trying to come up with a way to be able to boot to Mojave from Mountain Lion since it cannot see the volume (due to it being APFS)...  Is there anything I can do?  The last thing I want to have to do is solder wires to the mother board and connect them to a switch so I can disable a drive--- I would much rather find a way to do this with software.
Create a small JHFS+ formatted partition. Install the rEFInd Boot Manager to this partition. Configure rEFInd to silently boot Mojave. If configured properly, rEFInd will appear in the Startup Disk pane of System Preferences of Mountain Lion. You then can select rEFInd from Mountain Lion. When rEFInd boots, the software will silently instruct the firmware to boot Mojave.
A simpler alternative may be to rename the System/Library/CoreServices/boot.efi  file in the Mountain Lion startup volume. The may prevent Mountain Lion from booting, which may result in Mojave booting.