I have an iMac14,2 with a busted display I decided to use to test Catalina DP8 on. I've been using it with the Apple TB Display with no real issues, but when I tried to install Catalina, it rebooted about a dozen times (which I figured wasn't unexpected) and at some point, the machine shut off in a way that made it obvious something had gone wrong.
I was installing it on an external SSD connected via USB, and I didn't touch the startup security settings (I didn't think it mattered for non-T2 macs)
I think it was either booting or installing with the Apple logo with the progress bar, then just crashed after a glitch or a flash on the display. You probably know what I'm talking about, it sometimes happens when you press and hold the power button to force a shutdown, or when kernel panics.
So I thought I'd just have to do the install over again and tried to boot the iMac, except it wouldn't. Nothing showed up on the TB display, and I thought it was just a boot display problem so I tried a DVI Cinema Display with the mDP-DVI adapter, and then an analog panel with an mDP-VGA adapter, nothing was working.
Then I noticed that I hadn't heard the startup chime for a while now when I tried to boot the machine, and I did an SMC reset, removed half the RAM, got a wired keyboard and connected it to an ethernet drop to try to do a NetBoot or Internet Recovery, but it isn't even sending out bootp requests.
So at this point, I'm thinking this machine is "bricked" though I'm not really sure if the lack of a built in display has something to do with it. Since I can't really do anything about that, I'm assuming that's not the issue, and the only other thing I can think of is the install failed during an SPI ROM or SMC firmware update. Was there a BootROM or SMC update included in the Catalina installer? And is there any way to recover the firmware if it is corrupted?