I have an old 2012 and am having SMART errors so it's upgrade time. I'm still on High Sierra. Indeed, I can't upgrade my internal drive to Mojave because of the SMART errors. I'm basically planning to do a clean install but, until I get everything set up, I'd like to be able to essentially run the system from an external SSD via USB 3. My thought is that I'll use Carbon Copy Cloner to create a clone of my internal drive. Then upgrade that to the most current version of Mohave. I'm assuming I can still boot from that drive on a 2019 iMac? That would make my life easier and let me relax a bit when getting everything installed and up and running. Any other tips here would be much appreciated. Thanks!