Because of the mobility of my work I decided to create a boot volume on an external USB-C SSD so that I can run my user environment on a number of Macs (on an iMac Pro (2017) while at work, on a 15" MacBook Pro (Retina, mid-2015) while on the road, and on a 27" iMac (2017) when at home). Unfortunately, while the 27" iMac and 15" MacBook Pro boot up fine from the external drive, the iMac Pro doesn’t.
I’ve tried resetting the NVRAM and even the SMC for good measure. The external boot volume contains macOS Mojave 10.14.1, and this should be fully compatible with the iMac Pro.
What am I missing here?