How to downgrade from Tahoe 26.6 to Sequoia 15.7.9 on M1 machines without format issues?

On an Apple silicon Mac, moving from macOS Tahoe 26.6 back to macOS Sequoia 15.7.9 normally requires erasing the current macOS installation and performing a clean installation.

Sequoia 15.7.9 is an official Apple release, so if that is the specific version required for your company’s M1 Macs, you can obtain the full installer from Apple. Apple: How to download and install macOS

Before erasing the Mac, make sure all required data is backed up and, because these are company-owned Macs, confirm that your organization's MDM or IT policies permit installing Sequoia 15.7.9.

You can check whether Apple makes the specific installer available by opening Terminal and running softwareupdate --list-full-installers; if 15.7.9 is listed, download it with softwareupdate --fetch-full-installer --full-installer-version 15.7.9.

If you need to install Sequoia on multiple Macs, create a bootable installer from the downloaded Install macOS Sequoia.app using Apple's documented procedure. Apple: Create a bootable installer for macOS

To install it, shut down the M1 Mac, hold the power button until Loading startup options appears, select Options > Continue, and open Disk Utility. Select the internal startup disk and use Erase Volume Group, with APFS as the format. This erases Tahoe and prepares the Mac for the clean Sequoia installation. Apple: Erase and reinstall macOS

Quit Disk Utility and install Sequoia 15.7.9 from the bootable installer. After installation, complete Setup Assistant and restore your files and compatible applications. Rather than restoring the entire Tahoe system onto Sequoia, use Migration Assistant or restore your personal data selectively. Apple: Transfer information to Mac with Migration Assistant

If the Mac refuses to install 15.7.9 or you encounter an error while erasing or installing, provide the exact M1 Mac model and the exact error message. That information is important because the correct recovery or installation procedure depends on the particular Mac and the error encountered

Reverting to an older major version of macOS is not simple. You need to boot into Recovery OS, erase your internal drive, then install the older macOS, then restore from a backup made prior to the recently applied upgrade*. Even then, the firmware on the Mac may have been updated, meaning potential conflicts and a possible need for DFU mode and a Configurator revive/restore, which would need to be done via another Mac connected to yours. If that sounds complicated and risky, then you're assessing the situation correctly. Also note that in no case can you install a version of macOS that is earlier than the version installed on your Mac when it came out of the box. 

*Regarding the backup, if you want to pick up where you left off on the old version, you really need a backup made before the upgrade. If you do not have such a backup (why not?), then there are alternatives but you must be prepared to accept some work to get back to your normal. If you're in that situation, here are some strategies (and caveats):

• Manual file copying: Copy your most important files, folders, photos, and music to an external storage drive.
• Third-party backup software: Tools for backing up a Mac, such as SuperDuper or Carbon Copy Cloner can create a separate copy of your data. Do not assume that a clone created under the new major version of macOS will be bootable on your Mac.
• Cloud synchronization: Make sure cloud storage services such as iCloud, Dropbox, OneDrive, or Google Drive have finished synchronizing. Do not assume that every app’s data is stored in the cloud, as some applications store data only locally.
• Time Machine: If you have a Time Machine backup from before you upgraded to Golden Gate, you can use it to transfer compatible data after reinstalling Tahoe. When setting up the Mac from the backup, be sure to use a backup date from before you applied the major upgrade. You cannot restore from a backup made post-upgrade, and any changes since the upgrade will be lost.

Downgrading an Apple Silicon Mac to macOS  15 Sequoia  

On Apple Silicon Macs, downgrading to an older version of macOS requires a complete erase of the internal drive. 

You cannot install an older macOS version over a newer one. 

Before you begin, create a complete backup of all your data.

Step 1: Back Up Your Mac

This is the most critical step. Downgrading will erase your Mac's internal drive, so you must back up your files and settings beforehand.

Time Machine is the recommended backup method. Use Apple's built-in Time Machine to create a full backup on an external storage drive. 

Alternatively, you can manually copy all essential files to an external hard drive or another storage location.

Step 2: Create a Bootable macOS 15 Installer

To install the older operating system, you'll need a bootable USB installer.

On a Mac compatible with macOS 26 , download the Install MacOS Sequoia app from the Mac App Store.

Connect a USB flash drive with at least 15 GB of free space. 

Open Terminal, which can be found under Applications > Utilities, and run the following command:

Sequoia macOS 15 via Terminal 

sudo /Applications/Install\ macOS\ Sequoia.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume

Replace MyVolume with the name of your USB drive.

Step 3: Erase Your Mac's Internal Drive

Shut down your Mac completely. Press and hold the power button until the Loading startup options screen appears.

Click Options, then click Continue. Select Disk Utility and click Continue.

In the Disk Utility sidebar, select your internal drive, typically named Macintosh HD, and click Erase. Enter a new name for the volume if desired and make sure the format is set to APFS. Click Erase Volume Group to erase the internal drive.

Step 4: Install macOS 15 from the Bootable Installer

After the erase is complete, quit Disk Utility to return to the Recovery menu. 

Choose Install macOS Sequoia and connect to Wi-Fi if prompted.

Follow the on-screen instructions and select the internal drive you just erased as the installation destination.

Wait for the installation to finish. 

Your Mac will restart several times during the installation process.

Step 5: Restore Your Data

After MacOS Sequoia has been installed and the initial setup is complete, restore your data. 

If you backed up your files manually, copy them back to their appropriate folders.

Important Considerations

Data loss: Any files created or modified on macOS 26 after your last backup will be permanently lost during the downgrade.

Application compatibility: Some applications updated for macOS 26 may not work properly on macOS 15.

Hardware compatibility: Your Mac must support macOS 15. A Mac that shipped with macOS 26 and never supported macOS 15 cannot be downgraded.

Firmware considerations: Downgrading to a significantly older macOS version may or may not require a firmware downgrade, which increases the complexity and risk of the process. 

Apple Configurator can be used for more advanced recovery and downgrade scenarios.

User Risk Assessment: macOS Downgrade Procedure

Before initiating any downgrade from macOS 26 Tahoe to an earlier version such as macOS Sequoia, assess the user’s tolerance for potential risks. The following checklist must be completed before proceeding.

Primary System Dependency

Determine whether the affected Mac is the user’s primary or only computer.

If the user cannot function without this system for several days in the event of a failed downgrade, the risk level is high.

The downgrade should be postponed or avoided until proper contingency measures are established.

Availability of a Secondary macOS System

Confirm that the user has access to another Apple computer currently running macOS 15 or macOS 26 (Tahoe).

This secondary system must be capable of performing a Revive or Restore operation on the target Mac’s firmware if required.

Lack of a secondary Mac significantly reduces the ability to recover from firmware corruption or boot failures.

Firmware Compatibility and Secure Enclave Risks

Be aware that upgrading to macOS 26 (Tahoe) likely includes a firmware update affecting the logic board and Secure Enclave Processor (SEP).

Downgrading to an earlier version (e.g., Sequoia) may or may introduce firmware mismatch issues, leading to instability or loss of functionality.

Backup Integrity and Recovery Preparedness

Verify that the user has at least three complete backups created before the macOS 26 (Tahoe) upgrade.

Implement the 3-2-1 Backup Strategy:

3 total copies of essential data

2 different backup methods or storage media

1 copy stored offsite (for protection against natural or human-caused disasters)

Each backup should reside on a dedicated, single-purpose external drive.

For enhanced backup and cloning capabilities beyond Time Machine, refer to:

🔗 https://bombich.com

Recommendation Summary

If the user answers NO to any of the above questions:

Downgrading the Mac is not recommended.

Proceeding without proper contingency plans, verified backups, and recovery tools may result in:

Data loss

Firmware corruption

Secure Enclave failure

Complete system inoperability