iMac Core i7 27" Mid-2011 EMC 2429 HDD went broken

Thanks again - I need to read answer properly later..

Last week I did change already few years ago broken ATI display adapter and last weekend (after earlier problems) also hard-disc. I was sissy not to go to SSD, so I did choose proved 2GB Seagate Barracuda.

Test Lion to Sierra installation did fail, but I did manage return to Lion, and then I did go to El Capitano. Since everything did work fine - it was stupid to try again High Sierra.

APFS came to my mind also - but I don't believe High Sierra installation would change that. I need to check that fact with Disk Utility.

I know how bad is Verbatim and such "USB's" are - so I did use few well working Kingston DataTraveller sticks.

Story continues. Thank you all helpers.

Many thanks to all who gave me some answers.. I did not answer many of them - since there was many.

There was also really many stirring web articles, vblogs telling different things.. if iMac was internally like crow's nest, web is another. Apples decision to cut OS support and change to ARM did make reading support articles even more difficult.

But you gave me result I have now. iMac is running High Sierra with half of RAM until I will find more.

Since I could not do anything with my iPhone and had only one broken Mac, making bootable media seem impossible and there was information that corresponding Mac needs to be compatible with needed bootable media. I got Macbook Air, but it did not seem to make any sense to downgrade that just for for making one bootable media. I am happy that Catalina was compatible enough - at first I didn't have courage to start installing old operating system inside Catalina, but it was needed.

Ten years ago, before I changed to Risk management, I maintained several flight safety equipments for 20 years, so this was my nice summer project and I did learn a lot.

Hopefully I find 1333 MHz DDR3 SO-DIMM (PC3-10600) answer - is there something special. Ones I watched, they sell them double..

I did combine my answer to you and user "Servant of Cats" earlier. I really hope still usable i7 iMac will survive. I could use it - if restoring it would not be made so impossible.