Create partition on APFS internal disk to download Catalina

The Catalina installer does not download as a .dmg file. It downloads as a standard app into the Applications folder titled something like "Install macOS Catalina". You can find links to various macOS installers as well as optional instructions for creating a bootable macOS USB installer:

How to create a bootable installer for macOS - Apple Support

You need to properly prepare the destination drive by using Disk Utility to erase the destination drive as GUID partition and APFS (top option). Then install macOS and select the newly erased drive as the destination for the installation of macOS which will make the selected drive into a full macOS boot drive.

Your USB stick ("/Volumes/Untitled") needs to be formatted as MacOS Extended (Journaled) in order to create the bootable macOS USB installer. The instructions in the Apple article and the error message in the screenshot clearly mentions this. You have not yet created a bootable macOS USB installer as evidenced by the posted screenshot.

After successfully creating the USB installer, you will need to Option Boot the Mac by holding down the Option key immediately after the startup chime in order to access the Apple boot picker menu where you need to select the amber icon which should be the macOS USB installer.

I'm not entirely sure what your goal is here as I am a bit confused by this thread. You have two choices when installing macOS. You can reinstall macOS over top of itself, or you can perform a clean install by first erasing a drive. You can install macOS to an internal drive or an external USB3 SSD. When installing Catalina, the destination drive needs to be GUID partition and APFS (top option).

I thought I’d put my money where my mouth is and go through the process that I suggested.

I split my SSD into two containers then installed Catalina on the new one.

All fine. I can boot at choice into Mojave or Catalina.

I didn’t need the three bootable backup clones of  Mojave which I am very happy about.

See screen grabs.

You must install Catalina to the APFS volume.

I'd make a bootable USB flash installer but I guess running the Catalina installer from Mojave should also work when you choose the new empty APFS volume as a target.

Backup before doing such major installs.

Which macOS boot drive (Mojave or Catalina) will need the most performance? And which will you use the most and for the longest time?

I definitely agree with the suggestion to use an external bootable macOS USB3 drive. Using multiple operating systems on a single drive is always risky & complicated usually leading to problems later on. Keep in mind that Mojave may complain about the Catalina APFS volumes (the "Update" volume in particular -- this can become annoying). Plus Mojave may not show the "Data" volume on the Catalina drive without manually locating the mount point in the Finder. This is because Mojave does not understand the new drive layout of multiple APFS volumes which macOS secretly & magically combines into a single "Macintosh HD" volume by default while booted into Catalina.

You may want to consider cloning your current macOS Mojave installation to an external USB3 SSD using Carbon Copy Cloner (CCC). You can use CCC while booted into Mojave, but just make sure to close all the apps and not do anything else when cloning the drive. Then you can either perform a clean install of macOS on the internal drive by first erasing the whole physical drive, or upgrade macOS over top of Mojave to preserve all your user accounts, data, settings, and apps.

Which OS you use on the internal & external drives is your choice based upon how you see yourself using each OS now & in the future.

Rich839 wrote:

I did format my USB flash drive Mac OS Extended (Journaled) and created a Catalina bootable installer on it,

Good, you did this part correctly.

but when I tried to boot up in Catalina by booting while holding down the Option key, the USB drive was grayed out and not selectable as a boot drive, with the notation that the drive is not an APFS drive.

You cannot select the USB installer as the destination since you are currently booted from it.

So I erased the USB drive and formatted it APFS. When I tried to create it as a boot drive for Catalina, terminal gave me the message that APFS drives cannot be used as bootable install media.

Most people use two drives and create a macOS USB installer using a USB stick while installing macOS onto a USB3 hard drive or SSD.

Is this a USB stick or a USB drive (hard drive or SSD)? If this external drive is a USB hard drive or SSD, then technically you can use it for both the installer and the full installation of macOS as long as the drive is properly prepared, but you will end up with at least 15GB - 20GB partition used by the installer. When preparing the USB drive you would need to create two partitions on the drive with one of them about 20GB (I think) to be used for the installer with the MacOS Extended (Journaled) file system and name it "Untitled" like before, and leave the rest of the drive as an APFS partition named "External Boot HD" which will be the destination for the full macOS install. When using the command line make sure to select the correct volume for the installer. When booted to the macOS installer, you will select the APFS volume called "External Boot HD" (or whatever you decide to name it).

If you are using a USB stick instead, then that is not a good option since most USB sticks are not large enough and most USB sticks are extremely slow especially for booting an OS (even a USB3 stick).

Lets go back to original query about partitioning your existing APFS internal disk.

Firstly make one or two bootable clones of your existing Mojave on an external drive(s). Check they are bootable then disconnect them from your MacBook.

Now you CAN partition an APFS disk.

Go to Disk Utilities, select the Disk not the container or volume then choose the menu option ‘partition’. Go through the procedure of creating a partition by using the +. Select the format of the new partition as APFS GUID.

It will create a new container and volume. The container is equivalent to a partition.

Your Mojave container/volume will be untouched except it will be restricted to a smaller size.

You should then be able to install Catalina on the new container/volume. I have never tried to install Catalina this way but I see no reason why it wouldn’t work. If you have one or more reliable bootable backups of Mojave then you can always go back to where you were or just simply delete the new container/partition.

Need_help_give_help wrote:

It will create a new container and volume. The container is equivalent to a partition.

Your Mojave container/volume will be untouched except it will be restricted to a smaller size.

You should then be able to install Catalina on the new container/volume. I have never tried to install Catalina this way but I see no reason why it wouldn’t work. If you have one or more reliable bootable backups of Mojave then you can always go back to where you were or just simply delete the new container/partition.

Yes, technically this should work, but I never recommend people do this because they almost always regret it later when trying to remove an OS and reclaim or resize the partitions because there is not enough room for either OS. Modifying partitions is always risky and almost always seems to lead to a temporarily unbootable system and potential loss of data (or inability to temporarily access data). Plus older versions of macOS will complain about the other unknown volumes associated with macOS 10.15+ and having multiple "Data" volumes can become very confusing for people.

It is much better and safer to install any other OS to an external drive. Pick the OS you use the least or needs the least performance to be located on the external drive although a USB3 SSD will be as fast or possibly faster than the internal Apple SSD on that laptop, but there is a higher risk of accidentally disconnecting the USB cable. USB3 SSDs are not that expensive and will give good performance for booting & using an OS.

Another option if performance and resources are not an issue is to install one of the operating systems into a Virtual Machine such as Parallels.