Cannot boot thumb drive on MBP 2017 to install Linux
up for one answer above, plus make sure that the adapter you're using is the right adapter for running the external hard drive's operating system. I had a 2015 MacBook Pro and I was running kali linux on an external hard drive. I was forced to buy the 2017 version since my dad needed a new computer and he wanted the 2015 MacBook I had. I mindlessly just picked out a USB-C to USB adapter, and that adapter worked for reading and transferring files from different hard drives, but when I try booting the kali linux hard drive, it would ask "which hard drive to boot" after pressing the option key, but it would end up having a black screen when I chose the kali. I almost deleted the kali linux operating system I had, and all the files in it (repeat/repartition drive, etc) but a light bulb came to my head and prompted me to check the box of that stupid adapter I bought...sure enough, it was only a USB-C to Type A USB. This may not be the same case for you, but my problem was solved when I bought a different adapter for the USB-C connection for the 2017 Mac.