iMac 2015 Retina dead after fiddling around with Thunderbolt 3 Adapter

[fabj]

I had the idea to attach a Thunderbolt 3 drive (or an external SSD with an USB-C interface) to my iMac 27" Retina Late 2015.

So I took my Apple TB3-TB2-adapter, a TB2-cable and a LaCie Rugged Pro Drive as well as an Icybox USB-C Case with an SSD containing bootable volumes and happily tried all ways of connecting, putting plugs in and out an into each of the two TB2-ports. Whithout success, though.

Restarting to double check whether it might eventually recognize an external drive, my Mac would suddenly start in installation mode trying to reinstall Catalina on the internal volume (running on Catalina). This happened twice and I shut down the computer both times (forced by long power button press first time and then by unplugging the computer from the socket).

Since then, black screen, no chime, no boot menu. Checked RAM and reset PRAM/SMC, disattached all external stuff, tried all keyboard shortcuts. When pressing power button, it starts, after a second restarts, after 10 seconds restarts again, then it remains.

Can see some white LEDs from RAM tray, can hear something spinning in the upper left corner, USB ports power external devices.

Any explanations, anyone?

 

[Bigwaff]

Boot from a Catalina installer external USB

 

[PaulD-UK]

Reset the SMC/MVRAM.

 

[fabj]

Boot from a Catalina installer external USB

Gave it a try (USB stick prepared using Dosdude's Catalina Patcher Tool). No reaction (as I'd have expected since no chime and boot options).

Reset the SMC/MVRAM.

Have done so frequently.

Any more hints?

 

[Fishrrman]

Take ALL the RAM out.
Unplug it from the wall.
Let it sit like that for 2 hours.
Then put the RAM back in, plug it in, and try again.

I suggest that you refrain from the tbolt connections for a while...

 

[fabj]

Take ALL the RAM out.
Unplug it from the wall.
Let it sit like that for 2 hours.
Then put the RAM back in, plug it in, and try again.

I suggest that you refrain from the tbolt connections for a while...

No change, unfortunately (using either of the bars and alternating slots).

When looking into the opened RAM tray, I can still see the LEDs (green, not white, btw) turning on, then briefly turning off after 1 sec and again after 10 secs, then they continue to light up.

I considered attaching another screen in case it's a screen interface issue, however that would require using the TB2 connection (as Minidisplayport).

Given I will open it up – what to look for?

 

[Bigwaff]

No boot chime? .. or do these models not boot chime?

 

[fabj]

Yes, usually they do boot chime, mine stopped.

 

[Fishrrman]

"This happened twice and I shut down the computer both times (forced by long power button press first time and then by unplugging the computer from the socket)."

I'm wondering if the firmware could be "damaged" in some way.
As in... an "interrupted" upgrade?

How does one "revive" damaged firmware?

 

[fabj]

"This happened twice and I shut down the computer both times (forced by long power button press first time and then by unplugging the computer from the socket)."

I'm wondering if the firmware could be "damaged" in some way.
As in... an "interrupted" upgrade?

How does one "revive" damaged firmware?

Sounds reasonable. I've had other machines with a logicboard suddenly dying, here it felt more like it strangulating itself gradually, so I am kind of optimistic that the hardware is fine.

So there is a Mac Configurator, but that is for newer T2 macs. Couldn't research any procedure yet how to do with an older machine.

 

[Bigwaff]

I think this is your iMac, however, it certainly sounds like the "fix" here isn't working for you -

Firmware updates and the iMac Retina 5K 27-inch Late 2015 (iMac17,1)

What is wrong with this iMac, which has such a struggle updating its firmware? And why is Apple not acknowledging the problem?

eclecticlight.co

Could this fix firmware updating in the iMac Retina 5K 27-inch Late 2015 (iMac17,1)?

Many iMac17,1 computers are still stuck with firmware dating back to before macOS Catalina. Could this new approach enable them to install firmware updates again?

eclecticlight.co