How to choose the startup volume

Follow these steps:

1. Turn on, or restart, your Mac.
2. Immediately press and hold the Option key. After a few seconds, the Startup Manager should appear. The Startup Manager scans for available volumes.

Press C during startup
Start up from a bootable CD, DVD, or USB thumb drive (such as OS X install media).

Press D during startup
Start up in Apple Hardware Test (AHT).

Press Option-Command-P-R until you hear startup sound a second time.
Reset NVRAM

Press Option during startup
Start up in Startup Manager, where you can select an OS X volume or network volume to start from.

Press Eject, F12, or hold the mouse or trackpad button
Ejects any removable media, such as an optical disc.

Press N during startup
Attempt to start up from a compatible network server (NetBoot).

Press T during startup
Start up in Target Disk Mode.

Press Shift during startup
Start up in Safe Boot mode and temporarily disable login items.

Press Command-V during startup
Start up in Verbose mode.

Press Command-S during startup
Start up in Single-User mode.

Press Option-N during startup
Start from a NetBoot server using the default boot image.

Press Command-R during startup

Start from the OS X Recovery System¹