To select a startup disk in OS X (macOS), open System Preferences and click Startup Disk, then choose the volume you want to boot from. This sets the default startup disk for the Mac, and you can also pick a temporary one by holding the Option key during startup.
What Is a Startup Disk?
A startup disk is the volume (internal or external) that contains a bootable copy of macOS or OS X. It holds the system files, kernel, and essential software needed to start the Mac. Most Macs have a single internal SSD or HDD as the startup disk, but you can also boot from an external drive, a network volume, or a USB installer. The Startup Disk pane in System Preferences is the official way to set which volume the Mac uses by default.
How to Change the Startup Disk in System Preferences
- Open System Preferences from the Apple menu (or Launchpad).
- Click Startup Disk (it may be under the System section).
- Unlock the pane by clicking the padlock icon and entering your administrator password.
- Select the volume you want to use, such as your internal drive or an external SSD.
- Click Restart… to apply the change immediately, or close the window to save the setting for the next reboot.
The selected disk becomes the default startup disk for all subsequent restarts. If you choose an external drive, keep it connected at startup; otherwise, the Mac may fall back to the internal disk.
Using the Option Key at Startup
To choose a startup disk temporarily (without changing the default), restart the Mac and immediately hold the Option (Alt) key. After a few seconds, the Startup Manager appears, showing all bootable volumes. Click one to boot from it. This is useful for testing another OS, booting from a USB installer, or troubleshooting. The selection only applies to that boot; the default remains unchanged.
When You Might Need to Select a Different Startup Disk
- Booting from an external drive to run diagnostics or recover data.
- Installing a different version of macOS from a USB installer.
- Running Windows via Boot Camp (the Windows partition appears as a startup disk).
- Recovering from a failed internal drive by booting from a backup.
- Testing a new OS without replacing your main system.
Common Problems and Troubleshooting
If the Startup Disk pane is grayed out, you may not be an administrator or the pane is locked. Click the padlock and enter an admin password. If a desired volume doesn’t appear, ensure it is properly connected and formatted as a Mac OS Extended (HFS+) or APFS volume with a valid macOS installation. On Apple Silicon Macs, the Startup Disk utility works differently; you may need to use the startup options (hold the power button) to select a volume. If you see a prohibitory sign (circle with a slash) after selecting a disk, the system files may be corrupted; try booting into Recovery Mode (Command+R) and reinstalling macOS.
Comparing Startup Disk Methods
The System Preferences method sets a persistent default, while the Option-key method gives a one-time choice. There is also the startup disk command in Terminal (using systemsetup) for advanced users, but it requires sudo privileges and is less common. On Intel Macs, you can also use the Startup Manager by holding the Option key; on Apple Silicon, you press and hold the power button to access startup options. The Startup Disk pane is the most user-friendly for most people.
Costs and Safety Considerations
Selecting a startup disk itself is free and safe. However, ensure you have a backup before changing startup disks, especially if you’re booting from an external drive. If you choose a disk that isn’t bootable, the Mac will either show an error or fall back to another volume. Always verify that the volume contains a valid macOS installation. If you’re using a third-party external drive, ensure it’s powered properly (some large drives need their own power supply). Avoid disconnecting the drive while it’s in use.
Frequently Asked Questions
Why can’t I see my external drive in Startup Disk?
Make sure the drive is connected and powered on. It must be formatted as Mac OS Extended (Journaled) or APFS, and contain a bootable macOS installation. If it’s a Windows-formatted drive (NTFS or FAT32), it won’t appear. Also, check that the drive is not failing; a damaged drive may not be recognized.
How do I select a startup disk on Apple Silicon Macs?
On M1/M2/M3 Macs, shut down, then press and hold the power button until “Loading startup options” appears. Click Options, then select a volume. You can also use System Settings > General > Startup Disk (on newer macOS versions) to set a default.
Can I select a network startup disk?
Yes, if your Mac supports NetBoot and the network has a bootable image. In the Startup Disk pane, you may see network volumes if they are available. This is common in enterprise environments. For most home users, it’s not applicable.
What happens if I select a disk that isn’t bootable?
The Mac will show a question mark folder or a prohibitory sign. To fix it, restart and hold Option to choose a valid startup disk. If none are available, you may need to boot into Recovery Mode and reinstall macOS.
Does changing the startup disk affect my files?
No, selecting a startup disk only changes which volume the Mac boots from. Your files on other volumes remain untouched. However, if you choose a disk with an older macOS version, some apps may not work until you update.
Can I set a startup disk without an administrator password?
No, you need admin privileges to change the startup disk in System Preferences. If you’re a standard user, you’ll need to ask an administrator to unlock the pane for you.
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