Security Settings Do Not Allow This Mac to Use an External Startup Disk

Want to use an external startup disk but receive the "Security settings do not allow this Mac to use an external startup disk" alert message?

Many users are confused about what this popup message means and how to make it disappear. Relax! Follow this guide and you'll find out the answer.

Why Security Settings Do Not Allow This Mac to Use an External Startup Disk

This popup message means that you cannot boot your Mac with an external startup disk or USB.

The reasons are mainly:

• You have selected (or default setting) "Disallow booting from external or removable media" under Startup Security Utility Settings.
• OS is unknown source.
• OS can't be verified as legitimate.

Secure Boot Settings

This popup message has a great connection with Secure Boot Settings. Let's get a deep understanding of the Secure Boot Settings:

In total, there are mainly 3 types of Secure Boot Settings: Full Security, Medium Security, and No Security.

Full Security is the highest level of Secure Boot setting. Under Full Security, your Mac will verify whether your OS is unknown or legitimate. This secure boot setting ensures your Mac starts up from an OS that Apple trusts.

Medium Security relaxes some of the stringent requirements of Full Security while being stricter than No Security. It doesn't prevent your Mac from using an OS that is no longer trusted by Apple.

No Security almost has no restrictions concerning Secure Boot settings.

How to Fix Security Settings Do Not Allow This Mac to Use an External Startup Disk

After figuring out why this popup message appears, let's learn how to fix it.

All you need do is to change Startup Security Settings to what is written and instructed on the popup message.

Here is how:

1. Click Restart on this popup message.
2. Hold down Command + R to put your Mac into macOS Recovery.
3. Click Utility at the top left corner＞Startup Security Utility.
4. Enter Mac's password if asked.
5. Check No Security/Medium Security under Secure Boot.
6. Check Allow booting from external media under External Boot. (Allow booting from external or removable media under Allowed Boot Media)

   

7. Go to the Apple menu＞Click Restart＞Hold down the Option key.
8. Select the bootable disk.

Security Settings Cannot Be Changed

The popup message "Security settings cannot be changed" may be caused by:

• The disk is encrypted.
• System security policies.
• Permission restrictions.

Fixes:

• Disabling disk encryption via Disk Utility in macOS Recovery. (If you still got stuck in this message, have a try at turning off FileVault.)
• Use administrator privileges.
• Update software.

Startup Security Utility No Administrator Was Found

The reason why this error appears:

• No administrator account logged in.
• Permission issues.
• System problems.

Fixes:

• Switch to an administrator account.
• Check permission settings.
• Reboot your Mac.

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Conclusion

Are you clear about what to do while you receive this popup alert? Go and use the method in this post to allow your Mac to use an external startup disk now.