By default, your Mac starts up from its built-in hard disk, but a startup disk can be any storage device that contains an operating system compatible with your Mac. For example, if you install macOS on an internal or external drive, your Mac can recognize that drive as a startup disk. You can then follow the steps in this article to start up from it.
How to Find a Disk ID & Device Node Identifier in Mac OS X Command Line Aug 9, 2016 - 6 Comments If you need to find the disk ID or disk node identifier (like /dev/disk0s2) of a volume attached to a Mac for one reason or another, perhaps the easiest way to obtain this information from the command line is with the diskutil utility. The current Mac operating system is macOS, originally named 'Mac OS X' until 2012 and then 'OS X' until 2016. Developed between 1997 and 2001 after Apple's purchase of NeXT, Mac OS X brought an entirely new architecture based on NeXTSTEP, a Unix system, that eliminated many of the technical challenges that the classic Mac OS faced. OS X El Capitan, released during the Summer of 2015 and was available from the Mac App Store as a free download. Like previous versions of OS X, El Capitan has the annoying habit of automatically starting the installation process once the download is complete.
Use Startup Disk preferences
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When you use Startup Disk preferences to select a startup disk, your Mac starts up from that disk until you choose a different one.
- Choose Apple () menu > System Preferences, then click Startup Disk.
- Click and enter your administrator password.
- Select your startup disk, then restart your Mac.
If you see a message that your security settings do not allow this Mac to use an external startup disk, check the External Boot setting in Startup Security Utility.
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Use Startup Manager
When you use Startup Manager to select a startup disk, your Mac starts up from that disk once, then returns to using the disk selected in Startup Disk preferences.
- Press and hold the Option key immediately after turning on or restarting your Mac.
- Release the Option key when you see the Startup Manager window.
If your Mac is protected by a firmware password, you can release the key when you're asked to enter the password. - Select your startup disk, then click the arrow under its icon, or press Return.
If you press and hold the Control key during this step, your selection is saved in Startup Disk preferences, so it persists until you change it.
If your Mac is using OS X Lion 10.7.3 or later, you can also use this method to start up from your Time Machine backup disk. Startup Manager identifies your Time Machine backup as ”EFI Boot.”
If you can't select your startup disk or start up from it
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Check for these possibilities if you can't see your disk in Startup Disk preferences or Startup Manager, or your Mac won't start up from it.
Check for a valid operating system on the startup disk
Make sure that your startup disk is using a version of macOS that's compatible with your Mac. You might need to reinstall macOS on that disk.
Check startup security settings
If you're using a Mac that has the Apple T2 Security Chip, check the settings in Startup Security Utility. These settings determine whether your Mac can start up from another disk.
Check for Option ROM firmware
If you're in Startup Manager and can't see your startup disk, press Option-Shift-Command-Period. If that works, do this each time you want to start up from the device or from a startup disk connected to it.
To enhance system security, Mac computers with up-to-date software don’t show devices that have Option ROM firmware until you press these keys in Startup Manager to load the firmware. On Mac models from early 2015 or earlier, you can disable this security feature to load Option ROM firmware automatically. This removes an important protection against potential unauthorized access to your Mac by other people with physical access to it.
- Open the Terminal app, which is in the Utilities folder of your Applications folder.
- Type
sudo nvram enable-legacy-orom-behavior=1
, then press Return.
To undo this command, entersudo nvram -d enable-legacy-orom-behavior
.
If you're using a firmware password on your Mac, the ability to load Option ROM firmware automatically or manually is disabled as an additional security protection. If you want to remove this additional protection but keep your firmware password:
- Open the Terminal app, which is in the Utilities folder of your Applications folder.
- Type this command, then press Return:
sudo firmwarepasswd -setmode command -allow-oroms
To undo this command, entersudo firmwarepasswd -setmode command
. - Type the administrator password for your user account, then press Return.
- Type your firmware password, then press Return.
- Restart your Mac.
If you’re still using a Mac OS X the time will come when your computer won’t boot, or a problem may arise where you can’t take control of the device, and booting from an OS X installation media will be required.
This is why it’s recommended that you make a Mac OS X bootable USB when your Mac is in working conditions. However, if you find yourself on a scenario where your device (iMac, MacBook Pro, Air, Mac Pro or Mini) is not responding and you happen to have a Windows device, then you can still be able to make a USB bootable installation media for your Mac OS X to reinstall the operating system using the Recovery Assistant.
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These instructions will also work for Windows users, who are running Mac OS X on a virtual machine and need to upgrade to the latest version. For instance, to OS X Yosemite.
Things to know before proceeding
Before you dive into this guide, you’ll need a few things:
- A broken Mac computer with Mac OS X.
- A trial copy of the TransMac software.
- One high quality USB flash drive with 16GB of storage.
- A copy of Apple’s macOS (DMG file).
Now that you have all the necessary ingredients, you’re ready to make a Mac OS X bootable USB using the DMG file of the operating system with the steps below.
How to create Mac OS X bootable USB installation media
Before you can use TransMac, you may first need to partition your USB flash drive with a GPT partition, as a normal MBR partition may not work. To do this, you’ll need to use the Diskpart command-line utility on Windows.
Setting up GPT partition
Use these steps to set up a USB drive with a GPT partition:
- Open Start on Windows 10.
- Search for Command Prompt, right-click the top result and select the Run as Administrator option.
- Type the following command to open Diskpart and press Enter:
- Type the following command to determine the USB flash drive and press Enter:
- Type the following command to select the storage and press Enter:Quick tip: The
select disk 1
command as an example, but you have to replace 1 with the number of the flash drive you want to use. - Type the following commands to delete everything from the USB thumb drive and press Enter:
- Type the following command to convert the drive into a GPT partition and press Enter: Microsoft controller for windows 10.
- Type the following command to select the new partition and press Enter:
After you complete the steps, the USB flash drive from MBR to GPT format, you can use the steps below to create a bootable USB installation media to install Mac OS X.
Creating USB install media
Use these steps to create a bootable media to install Mac OS X:
- Download and install a copy of TransMac.Quick note: TransMac is a paid software, but it has a 15-day trial solution, that give us more than enough time to move the DMG files to the USB drive from Windows. (If you want to support the developer, you can purchase the full version.)
- Insert the USB drive that you’ll use to fix your installation of OS X. (Remember that all the data in the USB will be erased. Make sure you take off any important documents.)
- Right-click the TransMac software icon and Run as administrator. (You’ll be prompted to Enter Key or Run, because we’ll be using it once, click the Run option.)
- On the left pane, you’ll see all the Windows PC drives listed, right-click the USB drive that you’re intending to use to reinstall Apple’s OS X and select the Restore with Disk Image option.
- In the warning dialog box, click the Yes button.
- Use the Restore Disk Image to Drive dialog box to browse for the DMG file with the installation files for Mac OS X Yosemite in this case, and click the OK button to create a bootable USB of the operating system.Now, you’ll have to wait a long time. No kidding. It could take one or two hours to complete the process depending on your computer and other variables.
Once your bootable USB installation media is ready, remove it and insert it into your Mac, power it on, holding down the Option key, and select the USB you just created to reinstall Mac OS X.
If you’re having issues trying to create a bootable media, you can get a USB flash drive that comes with Mac OSX ready to install.
Update September 23, 2019: This guide has been revised to make sure it includes the latest changes.
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