How do I boot from an external USB device on a Macbook pro running OS-X Sierra? Unfortunately, I am new to Macs & OS-X, so am not sure how things work. I am trying to boot from an external USB drive containing Windows 10 on my Macbook pro running OS-X Sierra. However, when I start the system and hold the option key only the internal disk is shown and the option to net boot, no USB device is shown. The external USB drive boots successfully from a Linux PC and a Windows laptops.