The computer will start up from the chosen volume, but bear in mind performance will likely be much slower than when you normally operate your Mac. Use the keyboard arrows to choose your boot device, then press the Enter key. Continuing to hold this button down will bring up a menu where you can select a disc or drive to boot from.
For some reason recently, It has been locked to the first EFI partition (Which Is actually a separate drive carrying a separate test version of MacOS).
