Temporarily change your startup disk with Startup Manager

Startup Manager allows you to pick a volume to start from while the computer is starting up.

Use these steps to choose a startup disk with Startup Manager:

1. Turn on or restart your Mac.
2. Immediately press and hold the Option key. After a few seconds, the Startup Manager appears. If you don't see the volume you want to use, wait a few moments for Startup Manager to finish scanning connected drives.
3. Use your mouse or trackpad, or left and right arrow keys to select the volume you want to use.
4. Double-click or press the Return key to start up your Mac from the volume you selected.

If you have an optical drive connected to your computer, you can insert an installation disc to see it in Startup Manager. You can also attach FireWire or USB external hard drives that contain an operating system to add to the list of startup volumes. Startup Manager automatically adds bootable volumes as you connect them.