Mac mini Mac OS Base System Downgrade

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Level 1

8 points

Mac mini Mac OS Base System Downgrade

I have a Mac mini i7 2018 that shipped with Mojave. I recently updated it to Sequoia but found some incompatiblities with some software running on Mojave. I decided to go back to Mojave and used a Time Machine backup which worked but it kept giving me the message "some incompatible features on this disk" and also recovery mode doesn't work, only internet recovery. I believe the incompatible features is the recovery mode which has been updated to Catalina by Sequoia.

I have tried the CMD+SHIFT+OPTION+R but this only goes back to Catalina which is in the Mac OS Base System.

I would like to get the Mac OS Base System back to Mojave to get recovery mode working again. I've tried erasing the SSD (top level) when showing all drives but clicking Erase doesn't do anything, it only works on the Macintosh HD partition. Nor does it work when trying to erase the container on the SSD drive.

Is there a way to get back the Mojave recovery mode in Mac OS Base System? I'm aware many people will tell me to just update to Catalina or Sequoia or Tahoe but this is not an option. Mojave is the best system for my purposes and I don't need all the new features.

Thanks,

Darren.

Mac mini, macOS 10.15

Posted on Dec 21, 2025 11:39 AM

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Answer 1

Dec 21, 2025 12:07 PM in response to beaubadger

To downgrade the OS you'll first need to download macOS Mojave and create a bootable USB flash drive.

Download the older macOS version and create a bootable USB installer

When that is done, boot the Mac from that flash drive, and when the macOS Utilities window is presented,

a) Launch Disk Utility and format the internal drive. Select the GUID partition scheme, the APFS format and name the drive “Macintosh HD”. This step will erase all of your stored data from the drive. Quit Disk Utility.

b) Launch Reinstall macOS Mojave… and follow the prompts to select the destination drive - Macintosh HD - and begin the reinstall process. Reinstall macOS

If you use Time Machine to backup your Mac, then in order to recover your files you'll need to have backed them up to an external drive before you upgraded from Mojave. Otherwise, those TM backups may not be accessible to the Mojave system. If you're lucky you *might* be able to recover your files using a Finder window, but that may also severely compromise the backup set in the process.

Edit - forget the Mac OS Base System that Disk Utility displays. It is of no practical use. It resides only in the Mac's ROM for use by the computer in certain circumstances.

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Answer 2

Bigwaff

Level 1

23 points

Dec 21, 2025 12:07 PM in response to beaubadger

Good you have a Time Machine backup from when using Mojave system. You will have to create a Mojave USB install media and boot from the USB.

https://mrmacintosh.com/how-to-download-macos-catalina-mojave-or-high-sierra-full-installers/

Create a bootable installer for macOS - Apple Support

You'll need to boot into Internet Recovery and change Startup Security so you can boot from external or removable media.

Boot from Mojave USB media, open Disk Utility, make sure to select View > Show All Devices. Select your internal storage top-level device and erase. Make sure to select GUID partition and APFS. Install macOS Mojave. On first boot, try using Time Machine backup to restore. Sometime it work, sometime it not. If restore fail, just repeat the install process again (USB boot, erase storage, install macOS, etc) but skip restore and create new admin user. Once logged in as admin user, launch Migration Assistant and migrate from Time Machine backup.

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