`createinstallmedia` fails to create bootable volume for some Macs
I just experienced a similar, though non-identical, issue with my own 2012 Mac Mini which failed to start-up from a bootable High Sierra installer on a 64GB multi-boot flash drive. The drive is formatted with bootable installer partitions for 10.6/10.8/10.9/10.10/10.11/10.12/10.13/blank. I, too, was able to view all the partitions as start-up drive options, but the Mini refused to actually boot from any of them. And recovery mode’s reinstaller was forcing the internal start-up SSD to reformat as AFPS, which I did not want. I put the Mini in target disk mode &amp; connected it to my MacBook Air &amp; the Air to the flash drive &amp; used Terminal on the Air to install HFS+ High Sierra on the Mini. So, my take is that the issue is likely not related to details of the boot drive or its partitions but to something innate in the 2012 Mac Mini, itself.