Before you can turn the NetInstall service on you need to create a network disk image to boot from. The easiest image to create is a NetInstall image, which lets you install or update an operating system on any Mac.
Download a fresh copy of El Capitan
Before you can create a NetInstall image you need to make sure you have a fresh copy of the El Capitan installer downloaded to your hard drive.
If it doesn’t:
- Click Download.
- Click the App Store’s Featured tab.
- Locate and click OS X El Capitan in the Quick Links section of the App Store.
- Click the Download button for El Capitan.
- Sign in using your Apple ID if you’re asked to do so.
Create a NetInstall image
Once you have a fresh copy of El Capitan you can create your first image.
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Read more here:: How to create your first NetInstall Image