Thanks for the explanation. Based on your needs for this new iMac, I think the best way is to configure the volumes you need, macOS High Sierra and Ventura/Monterey, by first installing macOS High Sierra. Once macOS High Sierra is installed, you can add an APFS volume to your SSD, and install macOS Ventura, the latest version of macOS.
I've found it is best to first install the older version of macOS since Apple has designed modern macOS installers to support running from old versions of macOS, but not old versions of macOS Recovery interfacing with modern versions of macOS for tasks such as adding volumes, then installing to them.
Step 1: Create a macOS High Sierra installer, erase your disk, install macOS High Sierra, and migrate your data
First, create a macOS High Sierra bootable installer: Create a bootable installer for macOS - Apple Support. You will need a USB flash drive or other secondary volume with at least 14GB of available storage, formatted as Mac OS Extended.
After following the steps in the Apple Support article for downloading macOS High Sierra and using the Terminal to create the install media, follow these steps to startup from it, erase your Mac, and install macOS High Sierra:
- Plug the bootable installer into your Mac that is connected to the internet and compatible with the version of macOS you're installing. (A bootable installer doesn't download macOS from the internet, but it does require an internet connection to get firmware and other information specific to the Mac model.)
- Turn off your Mac.
- Turn on your Mac, then immediately press and hold the Option (Alt) key.
- Release the Option key when you see a dark screen showing your bootable volumes.
- Select the volume containing the bootable installer. Then click the onscreen arrow or press Return.
- Choose your language, if prompted.
- Select Disk Utility from the Utilities window, then click Continue.
- In Disk Utility, select View > Show All Devices. Select the top entry in Disk Utility titled "APPLE SSD...".
- Click Erase, and make sure the information matches:
- Name: Macintosh HD
- Format: APFS
- Scheme: GUID Partition Map
- Click Erase, and quit Disk Utility.
- Select Install macOS from the Utilities window, then click Continue and follow the onscreen instructions.
After installing macOS High Sierra, follow the prompts in Setup Assistant. When asked how you want to transfer your information, select the option to transfer from a Mac, Time Machine backup, or startup disk. Then click Continue. Connect your Time Machine drive, select your Time Machine backup, then click Continue. Continue to follow the steps to restore. Large transfers might need several hours to complete.
At this point, your new iMac should be running macOS High Sierra, with all your data restored.
Step 2: Add an APFS volume to your startup disk, and install macOS Ventura
After macOS High Sierra has been installed with your data migrated, you can add an APFS volume and install macOS Ventura, the latest version of macOS.
To start, follow the steps here, Use more than one version of macOS on a Mac - Apple Support, to add an APFS volume. Next, download macOS Ventura (or Monterey, if preferred): How to download macOS - Apple Support. When the installer opens and you're asked to choose where to install, click Show All Disks, then select the new APFS volume before continuing the install.
After your Mac restarts to the new version of macOS, follow the prompts in Setup Assistant. At this time, you can also restore data to this APFS volume, if you wish.
Jack
