Resetting Mac M1 and installing older Mac

For Apple Silicon computer >> Use Disk Utility to erase a Mac with Apple silicon.

Question 1 of your >>>

Use the Erase All Content and Settings feature to quickly and securely erase all settings, data, and apps, while maintaining the operating system currently installed.

Question 2 of yours >>>

Downgrading an Apple Silicon Mac to macOS 15 (Sequoia)

• On Apple Silicon Macs, downgrading to an older macOS version 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 (recommended): Use Apple's built-in Time Machine to create a full backup on an external storage drive.
• Manual copy: Alternatively, copy all essential files to an external hard drive or other storage location.

Step 2: Create a Bootable macOS 15 Installer

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

1. Download the installer

• • On a Mac compatible with macOS 15, download the Install macOS Sequoia app from the Mac App Store.

1. Create the bootable USB installer

• • Connect a USB flash drive with at least 15 GB of free space.
  • Open Terminal (Applications > Utilities).
  • Run the following command:

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

1. Shut down your Mac

• • Turn off the computer completely.

1. Enter Recovery Mode

• • Press and hold the power button until the Loading startup options screen appears.

1. Open Recovery Options

• • Click Options, then click Continue.

1. Launch Disk Utility

• • Select Disk Utility and click Continue.

1. Erase the internal drive

• • In the sidebar, select your internal drive (typically Macintosh HD).
  • Click Erase.
  • Enter a new name for the volume if desired.
  • Ensure the format is set to APFS.
  • Click Erase Volume Group.

Step 4: Install macOS 15 from the Bootable Installer

1. Quit Disk Utility

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

1. Select the installer

• • Choose Install macOS Sequoia.
  • Connect to Wi-Fi if prompted.

1. Choose the destination drive

• • Follow the on-screen instructions.
  • Select the internal drive you just erased.

1. Complete the installation

• • Wait for the installation to finish.
  • Your Mac will restart several times during the process.

Step 5: Restore Your Data

1. Restore manually

• • If you backed up files manually, copy them back to their appropriate folders after setup is complete.

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 require a firmware downgrade, which increases complexity and risk. Apple Configurator can be used for more advanced recovery and downgrade scenarios.

Hi, on an M1 Mac you normally need to enter Recovery, erase Macintosh HD in Disk Utility, and then reinstall macOS. If you want an older macOS, that may require a bootable installer, because Apple Silicon Macs do not always allow installing just any older version.

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