With my horror, I realized that after installing OS X Yosemite on an external hard drive, I could not boot back into Mavericks. What I saw was this flashing question mark:
Should this happen to you, just follow these simple instructions taken from :
- Turn off your Mac by pressing and holding its power button for a few seconds.
- Press the power button once to turn your Mac back on. Then, hold down the Option key on your keyboard until the Startup Manager appears.
- Select your startup disk from the list of drives that appear on screen.
- If your computer finishes starting up normally, choose System Preferences from the Apple menu. Then, click the Startup Disk icon in the System Preferences window.
- Select your normal startup volume (such as Macintosh HD) from the list of drives that appear in the Startup Disk window.