Recovery mode using external monitor /

andrewfromnew zealand wrote:

Thanks for replying!

You're welcome.

I'm using mirror mode without effect / Surely there is a way to select / force use of a screen?

No unfortunately there is not.

If not, then how do I disconnect the internal monitor?

Unfortunately that requires opening up the iMac, which is not recommended without the proper tools and skill set.

The other option, might be to leave macOS on the iMac and install Linux in a virtual machine.

Running GUI Linux in a virtual machine on a Mac | Apple Developer Documentation

https://www.quora.com/Can-you-run-Linux-in-a-virtual-machine-on-macOS

Maybe try setting the Displays in Mirroring Mode then go into Recovery Mode.

If that doesn't help...?

Then the iMac's built-in video display cable may need to be disconnected to force video out to the external display.

andrewfromnew zealand wrote:

imac pro 2013 / i7 / GTX780 / High Sierra 10.13.6 /

The only iMac Pros that Apple ever made came out in 2017. Those were all-in-one desktop computers with 27" 5K Retina screens and Intel Xeon W processors.

The iMac (27-inch, Late 2013) seems to match your description. You can update it to Catalina – but no further.

The screen is dead however the machine runs fine with an external mac monitor running from firewire port

That Mac does not have a FireWire port.