The process of creating the bootable OS X El Capitan installer causes the USB flash drive you're using as the destination for the installer to be erased. So, before you proceed, make sure you either have a backup of the flash drive's contents (if any) or that you don’t care that they'll be erased. If the Disk Utility app looks different from this description, you may be using an older version of the Mac OS. You can find instructions cloning a drive using an earlier version of Disk Utility. In the sidebar, select the volume to which you want to copy/clone data. The volume you select will be the destination drive for the Restore operation.
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If you're thinking about installing macOS Catalina, the first thing to consider is whether you want to install it directly onto your Mac or create a bootable drive. You can use a bootable drive on a hard drive partition for dual-software installation, to install on multiple Macs in your home, or as a bootable drive if you can't use the Internet Recovery partition.
Note: These instructions require the use of Terminal. If you don't feel comfortable making changes to your Mac with Terminal, you can create a bootable disk using the DiskMaker X program.
Before you start
Before you get started, make sure you have a thumb drive with at least 15GB of storage, or a spare external hard drive (one you aren't planning to use for anything else). You'll also need to download macOS Catalina and ensure that it's sitting in your Applications folder.
Note: After macOS Catalina has downloaded, it will automatically launch the installer to begin the installation process. Quit the installer when this happens.
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And please: Don't forget to back up your Mac before you do anything.
![]() How to format your external drive for macOS Catalina
You'll need to start with a clean thumb drive or external hard drive in order to make it a bootable drive.
Your thumb drive or external hard drive is now ready.
How to put macOS Catalina onto your external drive
Important: You will need to use an administrator account on your Mac in order to run the Terminal commands to create a boot drive. You'll also need to ensure that macOS Catalina is in your Applications folder and you'll need to know the name of the external drive.
The process could take a very long time, depending on the drive. When it is done, the Terminal window will report 'Done.'
How to install macOS Catalina with a bootable installer drive
Once macOS Catalina is installed on your external drive, you can install it on any Mac with the drive plugged into it. You can use this installer to upgrade your operating system easily on multiple Macs or to help downgrade if you decide you want to go back to an earlier version of macOS.
If you're downgrading from macOS Catalina, please check out this guide instead.
If you're upgrading to macOS Catalina, follow the steps below.
Questions?
Do you have any questions about how to create a bootable drive for the macOS Catalina installer? Let us know in the comments.
Updated September 2019: Updated for macOS Catalina.
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By default, Mac starts from its built-in hard disk, but a startup disk can be any storage device that contains bootable contents that compatible with your Mac. For example, if you install macOS or Microsoft Windows on a USB drive, your Mac can recognize that drive as a startup disk. This guide provides 2 ways to boot a Mac from a USB flash drive.
Requirement
Starting up your Mac from an external disk requires the following:
Let’s see how to boot a Mac from a bootable USB drive and what to do if your Mac doesn’t start up from it.
Way 1: Boot Mac from USB Drive using Startup Manager
Getting your Mac to load from a USB drive is fairly straightforward. Use the following steps, you can easily set Mac boot from an external drive in Startup Manager, so it’ll only boot from USB that one time.
Step 1: Insert the USB boot media into a USB slot.
Step 2: Turn on your Mac (or Restart your Mac if it’s already on).
Step 3: Press and hold the Option key immediately after you see the Apple logo. Holding that key gives you access to OS X’s Startup Manager. Once the Startup Manager screen appears, release the Option key. The utility will look for any available drives that include bootable content.
Step 4: Using either the pointer or arrow keys on the keyboard, select the USB drive you wish to boot from. Once selected, either hit the Return key or double-click your selection. The machine will start to boot from the USB drive.
Way 2: Set a Mac Boot from USB Drive using Startup Disk
When you use Startup Disk preferences to set Mac boot from an external drive, so it’ll boot from that disk until you choose a different one. Here is how:
Step 1: Go to Apple menu > System Preference, then click Startup Disk.
Step 2: Click the locked icon and then enter your administrator password.
Step 3: Select External drive as the startup disk, then restart your Mac.
What to do if your Mac does not boot from the selected drive
If you see a message prompts that your security settings do not allow this Mac to use an external startup disk, check the External Boot setting in Startup Security Utility, and then allow your Mac to use an external startup disk.
Step 1: Open Startup Security Utility.
Turn on your Mac, then press and hold Command (⌘) + R immediately after you see the Apple logo. Your Mac starts up from macOS Recovery. When you see the macOS utility window, choose Utilities > Startup Security Utility from the menu bar. When you’re asked to authenticate, click Enter macOS Password, then choose an administrator account and enter its password.
Step 2: Select “Allow booting from external media“.
If you want to select an external startup disk before restarting your Mac, quit Startup Security Utility, then choose Apple menu > Startup Disk.
Note: If you’re using Boot Camp in a dual-boot Windows/OS X environment, you may be unable to boot negatively into supported versions of Microsoft Windows XP, Windows Vista or Windows 10 operating systems installed on external USB hard drive.
Make sure disk has been formatted with a GUID partition type
Intel-based Macs support starting from an external USB storage device’s volume that has been formatted with a GUID partition type. If you wish to boot from the drive, it’s important to format the partition as “GUID Partition Table” rather than either of the other two ahead of time when you use that drive as a bootable drive.
Make sure your disk is bootable
Volumes that aren’t bootable and don’t contain a copy of a valid operating system aren’t listed in Startup Disk or Startup Manager. Make sure the external drive you’re trying to start from contains a usable operating system.
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