Adding HFS+ partition to APFS SSD drive

lovely272 wrote:

Thank you, but what worked in the end was to install Catalina on the main drive. Then add an HFS+ partition. Then use Recovery mode to install High Sierra on the new partition. Works. I can now use all my expensive Adobe and Microsoft software on the partition without being forced to pay a subscription for stuff I already own and works perfectly well.

I am a bit surprised that installing High Sierra did not forcibly convert the HFS+ partition into an APFS one. When upgrading to High Sierra, installing it would automatically convert the target drive to use APFS if that drive was an internal SSD.

Maybe it's because you used Recovery mode instead of the App Store installer.

OWC Rocket Yard Blog – Is It Possible Not To Convert To APFS When Upgrading To High Sierra?

First, I would delete that partition. I don't recommend partitioning any drive because sooner or later people realize that one or more of the partitions is too small....and the only way to fix it is by starting completely over.

A better option is to create a new APFS volume and install macOS into the new APFS volume. A new APFS volume behaves very much like a partition, but without needing to resize anything which also makes it much safer. Or, install the other OS onto an external USB3 SSD.

Use more than one version of macOS on Mac - Apple Support

In order to install an older version of macOS onto another drive or partition, you first need to create a bootable macOS USB installer using the instructions in the following Apple article:

Create a bootable installer for macOS - Apple Support

You may need to create a High Sierra USB bootable installer, boot from that, then install High Sierra to the partition. It would be simpler to create a High Sierra virtual machine and get rid of the partition.

Personally I don't se any reason to be running High Sierra.