How to use my 2008 iMac drive via a caddy but plugged into my 2017 iMac as if it was still in my 2008 iMac.

Thank you. So what you are saying is Monterey ( 2017 iMac ) will not (boot ) or (recognize) El Capitan

( 2008 iMac)to the extent that if I had for example OS Catalina on the removed ssd, then my 2017 iMac would boot or recognize it and I would then be able to use my 2017 iMac as a monitor and run externally my ssd as if it was still in my old iMac - So apparently I have to what find a 2008-2015 rig that will "boot" my removed ssd and use that rig as a monitor or just put my ssd in it and boot it up without the caddy ?

OK - So if I get a rig to do that what are the steps I would need to do to be able to use the new rig as a monitor and using the caddy with my ssd ?

Thanks in advance

Thanks BDAqua I am looking into that ...

Ok but I now instead tried plugging my ssd which is running Catalina I removed from my MacBook Pro into my 2017 iMac and the same thing happens. The ssd does not boot as an independent drive and allow me to use my 2017 which is running Monterey 12.3.1. as an independent monitor. With that said, I have seen YouTube videos depicting folks running separate drives into a common Mac computer as a host screen, some even run a drive with windows and the other drive is running Mac OS. So apparently, we're missing some steps here. Perhaps like when you are accessing your Time Machine backup drive. You have to switch from your regular host drive and have it recognize the connected auxiliary drive which in this scenario is one's Time Machine backup drive. Apparently just opening or attempting to boot up in Finder alone will simply list limited files and one is not able to actually run the operating system of the auxiliary drive selected.

" Independent monitor " ... I simply want to install my ssd hard drive removed from MY 2008 iMac, and by using the OWC external caddy I bought connect that into the back of my 2017 iMac via an usb 3.0 port, then be able to use this removed ssd from my 2008 iMac as an independent resident hard drive, all the while bypassing my 2017 OS but using it's monitor. The ssd from my 2008 iMac startup screen would appear just like as if it was still in my 2008 iMac.