Apple Wireless Keyboards will not recognize pressing and holding down startup keys during startup until the startup chime sound is heard. This is normal behavior. You may find it helpful to press and hold startup key combinations while the startup chime is playing. Two common startup key combinations are listed below.
Choosing a startup disk using the Option key
Press and hold the Option key while the startup chime is still playing.
You can also choose the startup disk manually by choosing System Preferences from the Apple () menu, then choosing Startup Disk from the View menu. Select your startup disk from the available choices and click Restart.
If you're using a removable hard disk drive to start up from, it must be powered on and connected to your Mac before it will appear as an available startup disk. If the startup disk you want to use is network-based, it must be available before it will appear in the Startup Disk System Preferences pane as a choice.
Press and hold the Command + Option + P + R keys while the startup chime is playing.