Chosen Solution
I’ve got a 2017, 21.5” iMac (EMC 3068) that will not boot. The hard drive was suspected to be corrupt and was the original HDD so I installed a new Samsung SSD, as I have 50 times before and used my Catalina USB installer, as usual, to install a new OS from. Holding the “Option” key brought the expected result and I was able to choose to boot from my USB installer. About halfway through the boot the computer just restarted and attempted to boot to the USB again. Over and over the computer restarts at the same place during bootup. I tried all 4 of the USB ports in the back of the iMac, always the same. I tried a second USB installer of Catalina that I created as a backup, same result. I was able to boot to Single User Mode (not sure what I could glean from that). I also connected to my WiFi to attempted to boot into recovery mode. I was able to download recovery, but about halfway into the boot it stopped again and restarted, same result. I was hoping someone could point me in the right direct. I’d like to swap out RAM sticks or maybe check the power supply or even re-flash the BIOS (or swap the IC all together). I am a very experienced technician and very capable with a soldering iron (IPC J-Standard, 610, and 620). Being pointed in the right direction would be helpful as I work on Macs as a hobby, my day job as an RF Engineer. Thanks in advance! Update: I tried to take a few screenshots in verbose boot while attempting to boot the USB installer.
OK thats good! So your issue is with the your internals drives load. You’ll need to pull it and then try running it from an external or a different drive internally. One rub here is if this is a Fusion Drive system (two drives HDD and a hidden SSD on the other side of the logic board. You’ll need to pull the SSD as well.
