Start up from macOS Recovery
- Connect the USB drive with the.iso file to your Mac system. From “Disk Utility,” click on your USB drive. Click on “Disk Image” and then on the “.” icon and select the.iso file that you have downloaded. Select “USB Drive” from the drop-down menu in front of “Type” and also select the name of your USB drive.
- After you install the new hard drive, you will need to boot to the recovery partition on the original drive if OS 10.7 or later is installed. If OS 10.6.8 or earlier is installed, you can use the original OS install discs that came with the computer. Power on the Mac while holding down the 'Option' key.
As long as you have a USB 3.0 or better it will be fast enough. When your external is plugged into the MacBook click the Apple Icon in the top left of your screen and a menu will show up, click 'About This Mac'. This will bring up a splash window with a circular pic of the Catalina Desktop and 6 tabs, Overview.
Determine whether you're using a Mac with Apple silicon, then follow the appropriate steps:
Apple silicon
Turn on your Mac and continue to press and hold the power button until you see the startup options window. Click the gear icon labeled Options, then click Continue.
Intel processor
Make sure that your Mac has a connection to the internet. Then turn on your Mac and immediately press and hold Command (⌘)-R until you see an Apple logo or other image.
If you're asked to select a user you know the password for, select the user, click Next, then enter their administrator password.
Reinstall macOS
Select Reinstall macOS from the utilities window in macOS Recovery, then click Continue and follow the onscreen instructions.
Follow these guidelines during installation:
- If the installer asks to unlock your disk, enter the password you use to log in to your Mac.
- If the installer doesn't see your disk, or it says that it can't install on your computer or volume, you might need to erase your disk first.
- If the installer offers you the choice between installing on Macintosh HD or Macintosh HD - Data, choose Macintosh HD.
- Allow installation to complete without putting your Mac to sleep or closing its lid. Your Mac might restart and show a progress bar several times, and the screen might be empty for minutes at a time.
After installation is complete, your Mac might restart to a setup assistant. If you're selling, trading in, or giving away your Mac, press Command-Q to quit the assistant without completing setup. Then click Shut Down. When the new owner starts up the Mac, they can use their own information to complete setup.
Other macOS installation options
When you install macOS from Recovery, you get the current version of the most recently installed macOS, with some exceptions:
- On an Intel-based Mac: If you use Shift-Option-Command-R during startup, you're offered the macOS that came with your Mac, or the closest version still available. If you use Option-Command-R during startup, in most cases you're offered the latest macOS that is compatible with your Mac. Otherwise you're offered the macOS that came with your Mac, or the closest version still available.
- If the Mac logic board was just replaced, you may be offered only the latest macOS that is compatible with your Mac. If you just erased your entire startup disk, you may be offered only the macOS that came with your Mac, or the closest version still available.
You can also use these methods to install macOS, if the macOS is compatible with your Mac:
- Use the App Store to download and install the latest macOS.
- Use the App Store or a web browser to download and install an earlier macOS.
- Use a USB flash drive or other secondary volume to create a bootable installer.
Built right into OS X, OS X Recovery lets you repair disks or reinstall OS X without the need for a physical disc.
The OS X Recovery Disk Assistant lets you create OS X Recovery on an external drive that has all of the same capabilities as the built-in OS X Recovery: reinstall Lion or Mountain Lion, repair the disk using Disk Utility, restore from a Time Machine backup, or browse the web with Safari.
Note: In order to create an external OS X Recovery using the OS X Recovery Assistant, the Mac must have an existing Recovery HD.
To create an external OS X Recovery, download the OS X Recovery Disk Assistant application. Insert an external drive, launch the OS X Recovery Disk Assistant, select the drive where you would like to install, and follow the on-screen instructions.
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When the OS X Recovery Disk Assistant completes, the new partition will not be visible in the Finder or Disk Utility. To access OS X Recovery, reboot the computer while holding the Option key. Select Recovery HD from the Startup Manager.