showing boot options on secondary monitor on iMac with broken screen

[0920887]

Hello,

I have a late 2013 iMac with a broken screen. I want to boot off a USB and install a new version of MacOS on it, however when I hold the option key and restart, the video output only shows on the iMac's screen, not my external monitor I have plugged in. I know when in MacOS I can hit option+F1 to mirror the displays, however this doesn't work on the boot options menu.

If I took the iMac's screen off and physically unplugged it from the inside would it concede and use my secondary monitor on boot (when I can select my boot device)? Is there any other way around this?

Thank you.

 

[Fishrrman]

I don't know for certain if this "boot behavior" can be changed, but... my -guess- is that it cannot be...

 

[0920887]

I don't know for certain if this "boot behavior" can be changed, but... my -guess- is that it cannot be...

I took off the screen and it changed the boot behaviour (for anyone else wondering the same). If you disconnect the internal display you can see the boot options on your external one. Why they block this until you physically unplug the internal display is anyone's guess. I have a new problem now though, when I turn on the mac without the display the fans go crazy and it runs under-clocked, which makes sense, there's a temp sensor in the screen. HOWEVER, when I plug the screen back in, the fans don't go crazy, but everything is EXTREMELY laggy, I mean you move the mouse, and it jerks over a few inches like 2 full seconds after you moved it. Getting through the log in is next to impossible. No idea why this is happening. When you unplug it it goes back to under-clocked and crazy fans.