Start Up Manager
Key Combination: Hold Option on start
Start Up Manager allows you to boot from an external device, whether that be a USB Flash Drive, External Hard Drive, or with more recent machines over the Network via Netboot. If your desired disk isn't showing up, newer machines don't show devices that use Option ROM Firmware until you load their firmware. To do so, press Option + Shift + Command + Period while in startup manager. If a firmware password is in use, the ability to load Option ROM Firmware devices is disabled.