No bootable device - insert boot disk and press any key When I boot my Macbook Pro 13" Retina (running newest Mavericks), it is showing me this error:


However! If I boot holding the ⌥ alt-key, it shows Mavericks as option. When I  press ⌤ enter-key to confirm, it will boot normally. (I'm using this Macbook now to type this question).

Why won't my Macbook boot anymore? I did not use disk utility or something. What can cause this issue?
Boot normally then open System Preferences.app > Startup Disk. Select your drive and reboot. This should bless the drive and make it the default when powering on.