iMac 2012 downgrade to High Sierra from bootable usb fails on last step

Well, it turned out to be something very simple and quite frankly quite stupid. I only had a 'real' Apple keyboard and no 'Magic mouse'.

All the time I used a bluetooth mouse, which was fine as long I was not trying to downgrade.

But as soon as I started that proces, the non-Apple mouse did not work and the Apple keyboard stopped working at the last step.

So I did the disk utility checks first (no problems there luckily) and then borrowed an Apple mouse from someone nearby. And that did the trick! Not immediately, I first had to 'get them to know each other' (meaning: start the iMac with Catalina, its current version) before starting the downgrade.

When I reached the end and - with the Apple mouse - I clicked on 'install on Apple' it said (....): "you cannot install this version because there's a newer one installed". Quite frustrated and not hindered by any doubts I used the disk utiliy - while still in the downgrade proces - to erase 'Apple' and then it started to install High Sierra. I am typing this while it still has 7 minutes to go, but I am optimistic that I will have a downgraded iMac.

And for someone who reads this when trying the same thing: this iMac from 2012 does not 'see' our Wifi (we have three different routers, probably too new) so I use the hotspot on mytelephone. Because once you start the downgrade proces, it cannot use an ethernet cable. It can only work with wifi.

sorry, I did not explain the background of why I am doing this. I want to use the old iMac as a monitor for my Windows laptop. This can be done with 'old' iMacs but you need two things: the right OS version & the right cable. Before I buy the cable I want to downgrade first. I contacted Apple support and it's all doable. So I know I followed the right steps (the links they gave me worked, but for the last step). If I try contacting them again I can't use chat anymore so I thought I'd ask here if anyone recognizes this.

hi,

thanx for the description. If I understand correctly you think the disk needs repairing before I can take that last step? I remember having done that several times, all in the proces of creating the usb stick and preparing the iMac. I remember also looking up al the possible ways of formatting the usb stick, and the MacOS Extended Journaled indeed was the way to go. But I will go back to running Disk First aid again, while leaving the usb intact for the moment (creating that takes some time, so I prefer to work on the iMac itself first).

Today I will receive the cable that I need to connect the iMac 2012 to my Dell & HP notebooks.

Interesting tour around cable webshop land to read about thunderbolt 1/2 and 3, which connector does this iMac have, think about the ports on my old and new notebooks (which ones will be free, which ones will work or not ...)

So I will come back here to report on the end result: can I use the iMac 2012 as a monitor for the Windows notebooks...?

My 90 year old mother needed a new iMac so after installing that for her, I took her old one home. It's so beautiful that I would really like to use it.

The question will be answered soon . . .