I have a 2017 27’ Imac with a 500gb SSD formatted in APFS , OS High Sierra 10.13.3 With my previous iMac (graphic card failed) I used an external 250gb SanDisk SSD formatted with Mac OS extended journal. This SSD was bootable using OS X Yosemite. When I connect this external SSD to my new iMac it appears in the start up disks preference but when I try to start up from the external SSD the screen goes dark and the computer freezes. The reason I want to start up from this external SSD is because it has a version of Adobe Photoshop which is not compatible with High Sierra and Photoshop is very expensive. Could this be a formatting issue and /or a High Sierra Issue ? Thank you for any assistance.