I got an #OCLP install of #macOS #Sequoia running on the 2012 iMac, using an external 120GB SATA SSD in a Thunderbolt 2 enclosure.
It "runs", but boy is it slow! BlackMagic Disk Speed Test reckons it's writing at ~130MB/s, and reading at ~170MB/s.
I split the internal Fusion Drive and get ~200MB/s write/230MB/s read from the SSD, 200MB/s read, 170MB/s write. I have a 512GB NVMe SSD in a USB3 enclosure and get 400MB/s r/w on that, but the iMac won't boot from it 😕

