2012 iMac refuses to boot usb

[Goldwinguk]

Title says it all. I have a late 2012 iMac that refuses to boot from usb. Doesn't matter the usb drive (I've used one that Ive installed High Sierra from on 3 other 2011's)
I old down the option key at boot and all I get is a blank screen. Basically it just sticks in the blank screen. It will boot into recovery so I'm at a loss. It's an old intel so firmware security has nothing to do with it I can see. Any ideas

 

[rm5]

I had something similar happen way back when I had my 2012 Mac mini, it would just not show the boot picker no matter what. I can't remember what I did... I think it might have just fixed itself after a few tries. Are you continuing to hold down Option until the boot picker appears? On my 5,1 Mac Pro, this can take up to 30 seconds, but I just keep holding down the key. Try that out and see if it works.

 

[Nguyen Duc Hieu]

Title says it all. I have a late 2012 iMac that refuses to boot from usb. Doesn't matter the usb drive (I've used one that Ive installed High Sierra from on 3 other 2011's)
I old down the option key at boot and all I get is a blank screen. Basically it just sticks in the blank screen. It will boot into recovery so I'm at a loss. It's an old intel so firmware security has nothing to do with it I can see. Any ideas

You can try select to boot from the USB flash drive from System Preference menu to see if it's still not possible.
If the problem is not with the USB flashdrive, then it must be with the iMac itself.
Normally, when an iMac refuse to boot from any other drives than the default drive, I would just think of firmware password (EFI password) because it's the default function of a firmware password.
But you are quite sure it's not the firmware, I run out of idea.

 

[Goldwinguk]

Ok few updates. Key combos on boot refuse to work at all.
Either hangs in a blank screen or just ignored the key pressed and boots.
I have booted to recovery mode before but nothing now.
When I did I get into recovery mode and opened firmware security it just asked me to switch on protection and set password. It's never asked me for any password when booting. Retrying it after I thought I'd set the p/w and it just asked me to turn on security and set password.

Keyboard and usb key checked on another iMac and work as expected.

Tested nvram didn't appear to do anything. Neither did reset of SMC.
Selecting usb drive from start up disk utility and you get the bless cannot set boot drive message.
Checked usb drive in disk manager > first aid comes up clear
Checked usb drive in info volume says bootable.

Out of ideas at this point, then tried one last thing. This imac had a broken screen. When I did get recovery mode working the broken screen was disconnected and the unit ran like a 2 legged dog with a kernel task taking about 99% CPU as expected.

Disconnected the broken screen and everything started behaving as normal (including the bios p/w)

Currently updating to Monterey and will report back

 

[Fishrrman]

OP:

I could not discern from your postings above.
Are you using a WIRED keyboard, or something else?

Can you boot to INTERNET recovery by using this key combo at bootup:
Command-OPTION-R

Will using this load the internet utilities?

If the computer boots and loads them, that validates the keyboard.

What I would try first:
- get a USB3 drive
- connect it to the 2012 iMac
- use disk utility to format it (as the internal drive is formatted)
- now use either CarbonCopyCloner or SuperDuper to clone the iMac's internal drive to the SSD
- once done, try booting from the SSD that way.

 

[Goldwinguk]

Yes original Mac usb keyboard

 

[Goldwinguk]

So I worked it out. Because the broken screen was disconnected it went into some sort of "limp home" mode. Basically once I got it up it had a kernel process battering the CPU and taking about 95% of processing.
Found if I connected the screen logic board (but strangely not the power) thing will run fine. Took the board out of the old screen before it went in the bin all updated to Monterey and running loverly