I recently bought a mac book air, and reinstalled the os, maveriks on it. But I notice there is a question mark over the start up disk icon in the system preferences. I am not sure what this means, as the mac seems to be running ok, I did a restart, etc & it seems ok. When I look at it in system prefs, it shows it highlighted in purple, and saying I have selected it as my start up disk. Is this anything serious?