Switch on the computer and iImmediately press-and-hold the <span class="keycap">Option</span> key (<span class="keycap">Alt</span> key) until a list of possible startup disks appears⌥</span> or <span class="keycap">Alt</span> key) when the startup sound is played.
Immediately press-and-hold the <span class="keycap">Option</span> key (<span class="keycap">⌥</span> or <span class="keycap">Alt</span> key) when the startup sound is played.