Preferências do Sistema → Disco de Inicialização é o que você quer?
Aseleçãodeveserpermanente.Tambémdêumaolhadana
This [manual] startup volume selection is temporary. To set the default startup volume for your Mac after your computer is running, open System Preferences and choose Startup Disk from the View menu (in Mac OS X) or open the Boot Camp control panel (in Windows).
Existem parece um certo atalho de teclado para tornar a seleção permanente a partir do tela de inicialização em si:
Boot your machine with the Option key down to invoke the Startup Manager, then hold down the Ctrl key prior to clicking on a volume to boot. (You may have to use the left and right arrow keys to select a volume first.)
If this trick is supported on your Mac, you’ll notice that upward-pointing arrow—the one that points to the selected disk—will change into a circular arrow instead.