Hi there! I have a 13" 2017 macbook pro, running High Sierra. I'm trying to boot a cloned Snow Leopard OS (Carbon Copy Cloner backup) from an external drive, but keep getting the 'Prohibited' / 'No Access' icon (see attached image) after I choose the drive in Startup Manager. I've tried this using a 4TB LaCie Thunderbolt RAID drive and a Samsung SSD T5, but neither get further than the No Access icon after Startup Manager. It works on my old 2009 13" MBP, but not on the 2017 MBP. Anyone any idea how I could fix this?