Switch on the computer and iImmediately press-and-hold the <span class="keycap"b>Option</spanb> key (<b>⌥</b> or<b>Alt</b> key) until a list of possible startup disks appearswhen the startup sound is played.
Choose the USB stick and press <b>Enter</b>. The USB stick appears as an external hard disk and might be labeled <b>EFI Boot</b> or <b>Windows</b> like in the following screenshot:
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[[!img install/inc/screenshots/mac_startup_usb.png link="noclass="screenshot" alt="Screen with the logo of an internal hard disk labeled 'Macintosh HD' and an external hard disk labelled 'Windows' (selected)"]]
[[!img install/inc/screenshots/mac_startup_usb.png class="screenshot" alt="Screen with the logo of an internal hard disk labeled 'Macintosh HD' and an external hard disk labelled 'Windows' (selected)"]]
[[!img install/inc/screenshots/grub.png link="noclass="screenshot" alt="Black screen ('GNU GRUB') with Tails logo and 23 options: 'Tails' and 'Tails (Troubleshooting Mode)'and 'Tails (External Hard Disk)'."]]
[[!img install/inc/screenshots/grub.png class="screenshot" alt="Black screen ('GNU GRUB') with Tails logo and 3 options: 'Tails' and 'Tails (Troubleshooting Mode)' and 'Tails (External Hard Disk)'."]]