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 chimesound 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.
Turn on your Mac, then press and hold <strong>Command(⌘)+R</strong> immediately after you see the Apple logo. Your Mac starts up from <a href="https://support.apple.com/kb/HT201314">macOS Recovery</a>.
To still protect your Mac from starting on untrusted external media, you can set a firmware password, available on macOS Mountain Lion or later. A firmware password prevents users who do not have the password from starting up from any media other than the designated startup disk.
<strong>If you forget your firmware password you will require an in-person service appointment with an Apple Store or Apple Authorized Service Provider.</strong>