Setting Your External SSD as the Primary Boot Drive on iMac (Late 2015) and Cleaning Up Extra EFI Partitions

If you’ve upgraded your iMac to macOS Sequoia on an external SSD with OpenCore Legacy Patcher (OCLP) and previously experimented with a Linux Mint dual-boot, you may have multiple EFI partitions cluttering your system. Here’s a guide to help set your external SSD as the main boot device and clean up unnecessary EFI partitions safely.

Step 1: Identify EFI Partitions and Mount the Correct One

1. Identify All EFI Partitions:

   • Open Terminal and list all partitions:

     bash
     Copy code
     diskutil list
     

   • Locate your external SSD and note its disk identifier (e.g., disk2s1 for the EFI partition on the external SSD).

2. Mount the Correct EFI Partition:

   • Mount the EFI on your external SSD (the one with macOS Sequoia installed):

     bash
     Copy code
     sudo diskutil mount diskXsY
     

     Replace X and Y with the disk and partition numbers of the external SSD’s EFI.

Step 2: Set the External SSD as the Default Boot Device

1. Open Startup Disk Preferences:

   • Go to System Preferences > Startup Disk.
   • Select the external SSD with macOS Sequoia as the default boot device.
   • Click Restart to confirm the change.

2. Configure OpenCore to Use External SSD as Primary Boot (if OCLP is installed on the external SSD):

   • Ensure OpenCore is correctly set up on the external SSD’s EFI.
   • Verify that config.plist is configured for your macOS installation.

Step 3: Clean Up Extra EFI Partitions

1. Unmount and Inspect Extra EFI Partitions:

   • Review the other EFI partitions listed from diskutil list.
   • Only delete partitions you’re sure are no longer needed, such as those associated with the Linux Mint installation.
   • To double-check, mount each EFI one by one:

     bash
     Copy code
     sudo diskutil mount diskXsY
     

   • Open Finder and inspect each EFI to confirm its contents.

2. Delete Unnecessary EFI Partitions:

   • To delete a redundant EFI partition, use Disk Utility or Terminal:

     • In Disk Utility, select the extra EFI partition and click Erase or Delete.
     • In Terminal, use:

       bash
       Copy code
       sudo diskutil eraseVolume "Free Space" diskXsY
       

     Replace X and Y with the disk and partition numbers you want to erase.

   • Warning: Avoid deleting the EFI partition associated with your macOS installation or any OpenCore configuration if needed for boot.

Step 4: Confirm Successful Boot Configuration

1. Reboot and Test:
   • Restart your iMac to confirm it boots from the external SSD by default.
2. Use Startup Manager for Verification:
   • Restart and hold Option (⌥) to open the Startup Manager. Check that the external SSD appears as the default option.

This process should set your external SSD as the primary boot drive, clean up extra EFI partitions, and prevent boot conflicts on your iMac. Let me know if you need further help with any steps!