Edit: After half day, my wife’s Mac started having the same problem. I have then applied the solution that I found in the Apple
Boot Mac from DVD, USB or External Drive
There might be some point when you want to boot up your Mac system from any drive except the main OS X disk. Apple has made it easy for everyone so you just need an easy keyboard shortcut.
Let’s suppose you have to use the installation disk that arrived with your system to format your hard drive and restore the factory settings. Or let’s say you are booting from the USB drive having a clean installation of Mac OS X for troubleshooting. You may even have a duplicate backup of Mac on another hard disk and you just want to confirm that it is bootable. All these reasons are good enough to boot from an external device.
The simplest method to boot to a device other than the internal hard disk is to push and hold Option key as soon as you hear the Mac startup music. If you keep holding it, it will show a menu from where you can choose a drive or disk for boot up. You can move using keyboard arrows and select the right boot device. Now press the Return key. Your system will boot from the selected volume.
But keep in mind that the performance of the computer will be slower than the normal operation of Mac. This goes especially for USB drives, which can slow down the booting process.
There are other ways as well. Instead of pressing option key, you can also press the C key. This is done when you are trying to boot up from a DVD or CD. This will avoid the menu, and just start from the disk. But this method will not work for FireWire and USB drives.
Booting to some other volume by using any of these ways is a temporary thing so you do not have to change any settings to go back to regular booting. Mac will switch to original boot settings the next time it starts.
Have you ever tried to boot from other media? Which method did you use? Send us your comments.
Tricks & Tips