This guide describes how to boot from an SD card.
1. Power off your BBB by physically disconnecting the USB/power cable.
2. Plug the micro SD card into the BBB’s micro SD card slot.
3. Insert the SD card into the SD card slot.
4. Hold the ‘Boot Button’ (S2) on the top right (near the SD card slot) and, while holding this button, insert the USB/power lead to connect the power.