iMac Running Big Sur 11.6 Won't Boot from External USB Drive on Older MacOS

iMac Running Big Sur 11.6 Won't Boot from External USB Drive with Older macOS: Solutions and Suggestions

You are using an iMac (Retina 5K, 27-inch, Late 2014) and trying to boot from an external USB drive with an older version of macOS, but it won't boot. Despite trying every possible method, you couldn't make it work. This article outlines potential solutions and suggestions for this issue. We will dive deep into the topic, covering key concepts and providing detailed explanations.

The Problem:

Users often try to downgrade their macOS version or create a separate bootable volume for testing or legacy application support. However, when using an external USB drive to boot an iMac with Big Sur 11.6, they might encounter issues when the iMac is equipped with the Apple T2 security chip. This chip was first introduced in the iMac (Retina 5K, 27-inch, Late 2017), but some late 2014 and late 2015 iMac models received T2 chip upgrades when serviced by Apple.

The Apple T2 Security Chip and External Boot Drives:

The T2 chip provides a secure boot process, ensuring that your Mac starts up from a legitimate, trusted operating system. When you try using an older version of macOS on an external USB drive, the T2 chip may prevent the iMac from booting from that drive due to the secure boot process. By default, the Secure Boot setting is configured to Full Security, preventing the iMac from booting from external drives with an older version of macOS.

Potential Solutions:

Although it may seem challenging, you can still boot your iMac with Big Sur 11.6 from an external USB drive containing an older version of macOS by modifying the Secure Boot settings or using alternative methods.

1. Change the Secure Boot settings:

You can change the Secure Boot settings in the Startup Security Utility. To access it, follow these steps:

1. Turn on your iMac and immediately Press and hold down the Command + R keys until you see an Apple logo or a spinning globe.
2. When the Utilities window appears, select Utilities > Startup Security Utility from the menu bar.
3. Enter your firmware password when prompted. (If you don't have a firmware password, leave it blank.)
4. In the Startup Security Utility window, change the Secure Boot setting to No Security.

After these modifications, try rebooting from the external USB drive again. Ensure that you have properly created a bootable USB drive, following the official guide from Apple, to minimize issues.

2. Use an alternative boot method:

If changing the Secure Boot settings is not an option, try booting your iMac in Internet Recovery Mode and then restart from your external drive.

1. Disconnect the external USB drive temporarily.
2. Turn on your iMac and immediately press and hold Option + Command + R keys until you see the spinning globe or Apple logo.
3. When the Utilities window appears, select Utilities > Startup Disk.
4. Select the external USB drive as your startup volume, and click Restart.

Summary:

• The Apple T2 security chip might prevent iMac models from booting from external USB drives containing older macOS versions.
• Changing Secure Boot settings in the Startup Security Utility might solve the issue; however, it could weaken the iMac's security.
• Alternatively, try using Internet Recovery Mode and then restart from the external drive.

References:

• Create a bootable installer for macOS
• How to use Startup Security Utility on your Mac