If you’ve installed an operating system on a drive but it isn’t listed, the volume you’re trying to start from might need repair.Volumes that don’t contain a valid operating system aren’t listed in Startup Disk or Startup Manager. Make sure you’ve installed an operating system, like OS X or Windows on the drive you’re trying to start from.

If you’re using an external drive, make sure it’s connected and turned on.If you don’t see the volume you want to start your computer from, check the following: Hold down the Command and R keys on your keyboard until you see the Apple logo appear onscreen.To start your Mac from the Recovery System, use these steps: You can also start your Mac from OS X Recovery or Internet Recovery if your Mac was manufactured after 2011. From the menu that appears, choose Restart in OS X.In Windows, click the Boot Camp icon in the system tray.If you have started up your Mac in Windows using Boot Camp, you can use the Boot Camp system tray to switch your startup disk default back to OS X. Startup Manager automatically adds bootable volumes as you connect them. You can also attach FireWire or USB external hard drives that contain an operating system to add to the list of startup volumes. If you have an optical drive connected to your computer, you can insert an installation disc to see it in Startup Manager. Double-click or press the Return key to start up your Mac from the volume you selected.Use your mouse or trackpad, or left and right arrow keys to select the volume you want to use.If you don’t see the volume you want to use, wait a few moments for Startup Manager to finish scanning connected drives. After a few seconds, the Startup Manager appears. Immediately press and hold the Option key.Use these steps to choose a startup disk with Startup Manager: Startup Manager allows you to pick a volume to start from while the computer is starting up. Temporarily change your startup disk with Startup Manager

The next time you start up or restart your computer, your Mac starts up using the operating system on the selected volume.

Select your startup disk from the list of available volumes.

Click the Startup Disk icon in System Preferences, or choose View > Startup Disk.

From the Apple menu choose System Preferences.

Set the default startup disk You can change the startup disk your Mac automatically uses from System Preferences.

Full list mac startup commands