Can't install Mojave on 2nd volume of new SSD
I installed a new SSD in iMac and yesterday I installed Catalina on 1 of 2 volumes. I want to install Mojave on the 2nd volume. This was the process I used yesterday to install Catalina without a problem:
However today when I try shutdown/start or restart holding down option key to display all volumes the only volume that displays is the volume with Catalina installed. There are 2 external drives and 1 USB installer that are all bootable (according to system preferences - startup drive options) but none of them show up so the software installation process stops there.
The 2 external drives and the USB installer can be selected as the startup drive in system preferences. If I select one of the external drives as the startup drive, close preferences, shut down and then do a normal startup the selected external drive will be the startup drive when the startup process is complete according to the "About This Mac" window and system preferences.
However, if I then restart or shutdown/start and hold the option key down, the internal volume seems to take over the startup process and shows as the only available drive so again the software installation process stops there. Continuing this startup process or shutting the mac off and starting up again results in the internal volume being the startup drive according to the "About This Mac" window even thought the external drive is still selected in the system preferences startup drive window.
Using Disk Utility I unmounted the internal volume and the data (?) volume thinking that on restart the internal volume would not be part of the restart process. I was wrong. It was the only volume that showed as available.
I'm out of ideas and need suggestions.
iMac 27″, macOS 10.15