external monitor not detected found working MacBook Pro 2020 Catalina 10.15.7 - when the usual advice doesn't work

This post is for anyone, especially with a new MBP, who can't connect to their external monitor unless the MBP boots in Safe Mode - in which case the external monitor is visible and operates normally. This means that the hardware is fine and there's a software problem .... somewhere.

In my case I am connecting a MBP 2020 with Catalina 10.15.7 to a Philips monitor using an Apple USB-C to VGA adapter (what can I say - the Philips is HDMI or VGA and I didn't have a USB-C to HDMI adapter) and to an AOC monitor using USB-C to Displayport. Neither monitor worked - can't detect in the Displays utility.

In my case, with a new MBP, I had transferred my old Mac files and applications to the new MBP using Migration Assistant from an external drive which held a Time Machine backup of the old MBP.

The standard advice on the web is to reset the non-volatile memory, reinstall MacOS, try your cables on other machines, etc. I tried all these but nothing worked. I found a LOT of posts where people received the same advice, and also had no success.

Apple Support suggested that I boot in Safe Mode, then launch each non-Apple application in the Applications folder while watching the external monitor. The idea is that one of the Applications would interfere with the monitor connection and I would see the monitor go dark. I tried this but no luck. Some applications won't launch in Safe Mode. And every app which did, was fine. At the end of the process I couldn't identify which application caused the problem, if any. But the monitors still didn't work in normal mode.

Eventually I went back to the beginning:

1/ factory reset (this means a disk erase followed by a clean MacOS install)

2/ on my old MBP, delete every application which is not provided by Apple. This felt painful but actually it's not so long as you still have rights to the applications. You can always download a new copy... and you don't need all the software on day 1; you can download when you need it.

3/ update the Time Machine backup for the old MBP

4/ Migrate the old MBP using the most Time Machine backup (which now has files created with non-Apple applications but no troublesome applications)

Maybe 45-60 mins to do this.

System works perfectly. I reinstalled Parallels on the new MBP, and Windows 10 and MS Office. No trouble.

At some point I may download an application and have an issue. But then I will know exactly which app creates the problem, which makes it a lot easier to fix. Especially with a recent Time Machine backup on hand.

Hope this helps someone.