Reinstalling MacOS on a sick iMac
I have a mid-2011 iMac (iMac 12,1) that recently seemed to have its memory fail (Crucial RAM that I installed 6 years ago). I ordered new RAM from OWC but it had the same problem. After spending an hour on the phone with their tech support they are going to replace the RAM again, but I would like to find out if there is something else wrong with the computer. I can occasionally get it to boot without the dreaded 3 beeps.
I tried to run Apple Diagnostics, but instead I got a splash screen for Internet Recovery that never did anything else.
I tried to run Apple Hardware Test from a bootable USB, but Disk Utility on my other iMac (running Big Sur 11.5.2) didn't like the disk images that I downloaded.
I tried to create a bootable installer for High Sierra (the latest OS available for that model), but I can't download the installer on my Big Sur iMac. It sends me to the Mac App Store which tells me there are no updates available for Big Sur. I had hoped to turn an external SSD into a new bootable disk.
How can I run diagnostics on the computer to find out if there is a problem other than the RAM? And how can I create a new bootable external drive to run Mac OS on that computer?