Hello, In the past, I have used an early 2015-MBP to boot up my external SSD via USB. Now, I am on a 2017 MBP running High Sierra and I can no longer boot up the SSD. I've tried two options; pressing "hold" during restart and selecting the SSD in the Startup Disk option. Both result in a circle with a slash through it. The ability to select the SSD is there in both cases but it won't boot. Has something changed since I'm using a new computer? Is there anything I can do get this working. Your help is very much appreciated!