Can't install Mojave on 2nd volume of new SSD

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Level 1

5 points

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

Posted on Aug 6, 2021 5:00 PM

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Answer 1

Best reply

HWTech

Level 9

56,384 points

Aug 7, 2021 5:03 PM in response to bherman

Did you install Mojave into the same Container as Catalina? I think you will need to create a new APFS Container in order to run Mojave alongside Catalina as opposed to creating just single APFS volume.

I really don't recommend running more than one OS on a drive since it usually leaves to problems (boot issues and data loss) because users always tend to choose the wrong sizes for the two partitions (or in this case two Containers). It would be better to install one of these operating systems to a dedicated external USB3 drive (assuming your Mac has USB3 ports).

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Answer 2

bherman Author

Level 1

5 points

Aug 7, 2021 5:30 PM in response to HWTech

I followed the instructions in "Disk Utility User Guide" from Apple. I created 2 volumes, each about 1TB. Neither is likely to become close full. Both volumes appear on desktop. I plan to replace the 2 external rather old backup HHDs with new SSDs. Each of these would be backup for the 2 internal volumes. Our iMac has 4 USB3 ports.

I'll look into the 2 container idea but hopefully someone can provide a suggestion on how to install Mojave on the 2nd volume.

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Answer 3

bherman Author

Level 1

5 points

Aug 7, 2021 9:11 PM in response to bherman

Found info on additional containers. I eliminated the 2nd volume on 1st container. Established a 2nd container. Both appear on desktop.

Unfortunately the problem I described in my first post remains. On my first attempt restarting & holding the option key all the drives were displayed and I selected the Install macOS Mojave thumb drive. The process got as far as the Mac utilities window and I selected the install macOS option. Then nothing happened for about 20 minutes so I quit macOS utilities and shut down the iMac. I've tried the option key again and the only drive showing up is the container/volume with Catalina on it. Using Recover only offers Catalina. Using Recovery with Shift-Option-Command-R offers the original OS - Yosemite.

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Answer 4

BDAqua

Level 10

240,275 points

Aug 6, 2021 5:19 PM in response to bherman

Quirky & opposite of my condition, 10.14.6 & 10.15.7 show in Startup Disk CP, except in 11 & 12 all 4 show up in Startup Disk CP, & all 4 show up using Option key at powerup !?

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