El Capitan Boot Disk

Create the disk as a CD/DVD. Open Control Panel. Click on Create a recovery drive. Click Create a system repair disc with a CD or DVD instead to create the disc as a CD or DVD and not as a USB flash drive, at the “Connect a USB flash drive” screen.
To create a bootable Mac drive you need any disk with Mac OS X 10.11.0 El Capitan or newer (10.12 Sierra, 10.13 High Sierra, 10.14 Mojave, 10.15 Catalina) either running as your main system, or just being installed on a drive that's connected to your Mac at the moment.

Disk size is sectors (931.5 GiB) MBR disk identifier: 0x7C170CB2. I had a Windows partition before I installed El Capitan, and now I can't boot into it.

These advanced steps are primarily for system administrators and others who are familiar with the command line. You don't need a bootable installer to upgrade macOS or reinstall macOS, but it can be useful when you want to install on multiple computers without downloading the installer each time.

What you need to create a bootable installer

A USB flash drive or other secondary volume formatted as Mac OS Extended, with at least 14GB of available storage
A downloaded installer for macOS Big Sur, Catalina, Mojave, High Sierra, or El Capitan

Download macOS

Download: macOS Big Sur, macOS Catalina, macOS Mojave, or macOS High Sierra
These download to your Applications folder as an app named Install macOS [version name]. If the installer opens after downloading, quit it without continuing installation. To get the correct installer, download from a Mac that is using macOS Sierra 10.12.5 or later, or El Capitan 10.11.6. Enterprise administrators, please download from Apple, not a locally hosted software-update server.
Download: OS X El Capitan
This downloads as a disk image named InstallMacOSX.dmg. On a Mac that is compatible with El Capitan, open the disk image and run the installer within, named InstallMacOSX.pkg. It installs an app named Install OS X El Capitan into your Applications folder. You will create the bootable installer from this app, not from the disk image or .pkg installer.

Use the 'createinstallmedia' command in Terminal

Connect the USB flash drive or other volume that you're using for the bootable installer.
Open Terminal, which is in the Utilities folder of your Applications folder.
Type or paste one of the following commands in Terminal. These assume that the installer is in your Applications folder, and MyVolume is the name of the USB flash drive or other volume you're using. If it has a different name, replace MyVolume in these commands with the name of your volume.

Big Sur:*

Catalina:*

Mojave:*

High Sierra:*

El Capitan:

* If your Mac is using macOS Sierra or earlier, include the --applicationpath argument and installer path, similar to the way this is done in the command for El Capitan.

After typing the command:

Press Return to enter the command.
When prompted, type your administrator password and press Return again. Terminal doesn't show any characters as you type your password.
When prompted, type Y to confirm that you want to erase the volume, then press Return. Terminal shows the progress as the volume is erased.
After the volume is erased, you may see an alert that Terminal would like to access files on a removable volume. Click OK to allow the copy to proceed.
When Terminal says that it's done, the volume will have the same name as the installer you downloaded, such as Install macOS Big Sur. You can now quit Terminal and eject the volume.

Use the bootable installer

Determine whether you're using a Mac with Apple silicon, then follow the appropriate steps:

Apple silicon

Plug the bootable installer into a Mac that is connected to the internet and compatible with the version of macOS you're installing.
Turn on your Mac and continue to hold the power button until you see the startup options window, which shows your bootable volumes.
Select the volume containing the bootable installer, then click Continue.
When the macOS installer opens, follow the onscreen instructions.

Intel processor

Plug the bootable installer into a Mac that is connected to the internet and compatible with the version of macOS you're installing.
Press and hold the Option (Alt) ⌥ key immediately after turning on or restarting your Mac.
Release the Option key when you see a dark screen showing your bootable volumes.
Select the volume containing the bootable installer. Then click the up arrow or press Return.
If you can't start up from the bootable installer, make sure that the External Boot setting in Startup Security Utility is set to allow booting from external media.
Choose your language, if prompted.
Select Install macOS (or Install OS X) from the Utilities window, then click Continue and follow the onscreen instructions.

Learn more

A bootable installer doesn't download macOS from the internet, but it does require an internet connection to get firmware and other information specific to the Mac model.

For information about the createinstallmedia command and the arguments you can use with it, make sure that the macOS installer is in your Applications folder, then enter the appropriate path in Terminal:

What you need to install Windows 10 on Mac

I have just wiped my iMac, and something must have gone wrong in the process as i cant change the boot drive by holding down option key and when i go to install reinstall macOS through macOS utilities when i select the 500GB hard drive the main one it just says 'Disk is locked'. The El Capitan installer is downloaded to the /Applications folder, with the file name Install OS X El Capitan. If you have already installed El Capitan and want to now create a bootable installer, re-download the installer from Apple.

MacBook introduced in 2015 or later
MacBook Air introduced in 2012 or later
MacBook Pro introduced in 2012 or later
Mac mini introduced in 2012 or later

iMac introduced in 2012 or later¹
iMac Pro (all models)
Mac Pro introduced in 2013 or later

The latest macOS updates, which can include updates to Boot Camp Assistant. You will use Boot Camp Assistant to install Windows 10.

64GB or more free storage space on your Mac startup disk:

Your Mac can have as little as 64GB of free storage space, but at least 128GB of free storage space provides the best experience. Automatic Windows updates require that much space or more.
If you have an iMac Pro or Mac Pro with 128GB of memory (RAM) or more, your startup disk needs at least as much free storage space as your Mac has memory.²

An external USB flash drive with a storage capacity of 16GB or more, unless you're using a Mac that doesn't need a flash drive to install Windows.

A 64-bit version of Windows 10 Home or Windows 10 Pro on a disk image (ISO) or other installation media. If installing Windows on your Mac for the first time, this must be a full version of Windows, not an upgrade.

If your copy of Windows came on a USB flash drive, or you have a Windows product key and no installation disc, download a Windows 10 disk image from Microsoft.
If your copy of Windows came on a DVD, you might need to create a disk image of that DVD.

How to install Windows 10 on Mac

Boot Disk For Windows 10

To install Windows, use Boot Camp Assistant, which is included with your Mac.

1. Check your Secure Boot setting

Learn how to check your Secure Boot setting. The default Secure Boot setting is Full Security. If you changed it to No Security, change it back to Full Security before installing Windows. After installing Windows, you can use any Secure Boot setting without affecting your ability to start up from Windows.

2. Use Boot Camp Assistant to create a Windows partition

Open Boot Camp Assistant, which is in the Utilities folder of your Applications folder. Follow the on-screen instructions.

If you're asked to insert a USB drive, plug your USB flash drive into your Mac. Boot Camp Assistant will use it to create a bootable USB drive for Windows installation.
When Boot Camp Assistant asks you to set the size of the Windows partition, remember the minimum storage-space requirements in the previous section. Set a partition size that meets your needs, because you can't change its size later.

3. Format the Windows (BOOTCAMP) partition

When Boot Camp Assistant finishes, your Mac restarts to the Windows installer. If the installer asks where to install Windows, select the BOOTCAMP partition and click Format. In most cases, the installer selects and formats the BOOTCAMP partition automatically.

Make El Capitan Boot Disk

4. Install Windows

Unplug any external devices that aren't necessary during installation. Then click Next and follow the onscreen instructions to begin installing Windows.

5. Use the Boot Camp installer in Windows

After Windows installation completes, your Mac starts up in Windows and opens a ”Welcome to the Boot Camp installer” window. Getting started with docker on machine. Follow the onscreen instructions to install Boot Camp and Windows support software (drivers). You will be asked to restart when done.

If the Boot Camp installer never opens, open the Boot Camp installer manually and use it to complete Boot Camp installation.
If you have an external display connected to a Thunderbolt 3 port on your Mac, the display will be blank (black, grey or blue) for up to 2 minutes during installation.

How to switch between Windows and macOS

Restart, then press and hold the Option (or Alt) ⌥ key during startup to switch between Windows and macOS.

How To Create An El Capitan Boot Disk

Learn more

If you have one of these Intel-based Mac models using OS X El Capitan or later, you don't need a USB flash drive to install Windows:

Create El Capitan Boot Disk

MacBook introduced in 2015 or later
MacBook Air introduced in 2017 or later³
MacBook Pro introduced in 2015 or later³

iMac introduced in 2015 or later
iMac Pro (all models)
Mac Pro introduced in late 2013 or later

To remove Windows from your Mac, use Boot Camp Assistant, not any other utility.

For more information about using Windows on your Mac, open Boot Camp Assistant and click the Open Boot Camp Help button.

1. If you're using an iMac (Retina 5K, 27-inch, Late 2014) or iMac (27-inch, Late 2013) or iMac (27-inch, Late 2012) with a 3TB hard drive and macOS Mojave or later, learn about an alert you might see during installation.

Os X El Capitan Boot Disc

Make El Capitan Install Usb

2. For example, if your Mac has 128GB of memory, its startup disk must have at least 128GB of storage space available for Windows. To see how much memory your Mac has, choose Apple menu  > About This Mac. To see how much storage space is available, click the Storage tab in the same window.

El Capitan Bootable Usb Disk Utility

Make El Capitan Boot Disc

3. These Mac models were offered with 128GB hard drives as an option. Apple recommends 256GB or larger hard drives so that you can create a Boot Camp partition of at least 128GB.