Hi, I just bought a new iMac and instead of using the fusion drive to boot and run applications/have "home," I want to use an SSD drive that I have in an enclosure and connected to the iMac. I am moving from a mid-2010 MacPro, where I had this same SSD as the boot drive, and the OS is High Sierra (as far as I could update that computer). Now... I have called Apple tech support and we tried to boot from that drive, but it just gave me the circle with a line through it. No go. That is literally as far as Apple was willing to go before sending the issue to the engineers. It's been a week and no one is getting back to me. I am certain this is doable, I spoke with Apple before buying the computer to make sure it was doable. I spoke with OWC where I bought the enclosure to make sure it was doable. I am guessing that the iMac isn't reading the older OS?? Maybe? So, I ask - anyone have any idea why the iMac won't boot from that SSD? Thanks in advance for any insight!