Booting from OS installer using second monitor
I’m attempting to decommission and reformat the HDD in a 2009 27" iMac with a failed display panel, using an external monitor. It can be coerced into using a second monitor as an extended or mirrored desktop (the computer is unaware that the display hardware has failed, as the graphics card still works fine), but when i boot from a High Sierra USB installer, the installer UI is stuck on the broken screen, with no apparent way to display it on the working extended display screen. Any ideas?
The I can think is maybe try to put it back into mirrored display mode and see if that sticks when booted into the installer. Sadly I don’t have the chain of adapters I might need to see if Disk Target mode works over thunderbolt 2 to thunderbolt 3.
Any suggestions, please let me know here or on Twitter @jeffcgd
Thanks to all.