Choose which disk or version of macOS your Mac starts from

Why it happens

Your Mac starts from whichever disk is set in Startup Disk, normally the built-in Macintosh HD. If you've installed macOS on an external drive or a second volume, or Windows with Boot Camp on an Intel Mac, you choose between them there. A Mac that keeps starting the wrong system usually has the other disk picked as its default.

How to fix it

1. Connect the external drive first, if that's the one you want. Then open the Apple menu, choose System Settings, click General, then Startup Disk.

2. Wait a moment while your Mac finds the disks it can start from. Click the one you want, enter your password if asked, and click Restart.

   General›Startup Disk

3. To pick a disk just once, on a Mac with Apple silicon: shut down, then press and hold the power button until you see Loading startup options. Click the disk, then Continue.

4. On an Intel Mac: turn it on and hold the Option key straight away until the disks appear. Click the one you want and press Return.

5. If an Intel Mac keeps starting Windows when you want macOS, hold Option as it starts and choose Macintosh HD. Then set Macintosh HD in Startup Disk so it sticks.

Still not fixed?

1. If your external drive isn't in the list, check it's plugged straight into the Mac and has a version of macOS installed that your Mac supports. A drive with only files on it can't start a Mac.

2. On an Intel Mac with the T2 security chip, starting from an external drive is blocked by default. Allow it in Startup Security Utility in macOS Recovery.

3. If the disk shows up but the Mac won't start from it, check it with First Aid in Disk Utility, then try again.

   Disk Utility›First Aid

4. If your Mac starts to a folder with a question mark, it can't find any working startup disk. Start up in macOS Recovery and run First Aid on Macintosh HD.

5. If the built-in disk doesn't appear at all, or Recovery can't repair it, contact Apple Support or a repair shop. The drive or another part may need a hardware check.