Fixed the issue: Security settings do not allow this Mac to use an external startup disk: when downgrading from macOS Big Sur to Catalina

2 weeks ago, I did the system update to install Big Sur on my work laptop. Running Selenium tests in Chrome was really slow and tests failed all the time.

It took about 4 mins to login to our portal. Very Slow. When using Safari to hit our portal, I got the same result.

Since this is the work laptop, I decided to downgrade from macOS Big Sur to Catalina. I watched many YouTube videos. This video is pretty good.

I followed the instruction, but I got the following screenshot.

Security settings do not allow this Mac to use an external startup disk

After looking into the issue, it was easy to fix it.

Went to Utilities -> Startup Security Utility. Logged into my credential.

Changed to Medium Security and chose Allow booting from external or removable media.

Restarted the machine using command + R. I finally saw the “Install macOS” option.

After installing Catalina, I set up everything for my Selenium Tests. Running the tests is very fast in Catalina, but not Big Sur. I am very happy to say that I “upgraded” from macOS Big Sur to Catalina on MacBook Pro successfully.