Happy New Year bogdaw and startergo.
Just caught up with your posts. You guys are brilliant! My task of refreshing old o/s and installing on 2011 iMac has today been finally completed after two years of technical hurdles. It has been a long journey.
Initially I purchased the 27" 2011 iMac in 2023 in working condition, because I needed old o/s to run software for my Artixscan M1 scanner (There is no professional quality consumer glassless scanner available these days that I know of) - and Epson Photo R2400 printer. After 2 months the graphic card failed. And then the power unit failed. I resurrected the GPU card by baking it. Six weeks later the card failed again and I put the 27" aside to concentrate on making a 2007 24" core duo Extreme do the work for me.
I upgraded the core duo with a 2tb SATA SSD and 6gb RAM and all was good. I had complete, previously cloned o/s Snow Leopard, Lion and Yosamite carefully saved on a couple of old LaCie drives with all applications and my old files. So I could use my scanner and printer with original software.
Meanwhile I vacillated about spending money buying risky used parts for the 2011 imac as the AMD Radeon 6970M was the known product of a partially faulty batch. In the end I made the effort, took a risk and purchased a used GPU and Power board which I have now installed and found good. So far!
I also transferred and installed the 2TB SATA for speed and started cloning the three o/s each onto its own partition. My reason for transferring all three o/s was because I had by now, entirely forgotten which o/s versions did or did not work for each external printer and scanner.
By February 2023 I wanted to clean install o/s over existing clones as glitches were occurring. I found Startego‘s post macOS Installer Links and USB Creation Guide on MacRumours and I persevered with the difficult task of dodging the Apple out of date certificate block which prevented clean installation from DMG.
This story is a complex way of saying a big ‘thank you’ for all the advice you guys have offered on MacRumours concerning boot drives and Legacy o/s transfer with Terminal. Somehow, thanks to you, I have also succeeded in adding Mountain Lion to the o/s selection and today have worked through and tested all the important applications as a prelude to weeding out unnecessary apps and files. It seems I can safely rely upon Yosamite for all the legacy operations but for the moment ,paranoia suggests I keep all four o/s on an external HDD for security, while retaining Yosamite as the working o/s. A great way to begin 2026!
If the hardware holds up I shall be a happy Bunny Rabbit.
My sincere thanks for your brilliant and valuable contributions.
