1. When finished, OS X El Capitan will boot into a brand new clean install, running through the initial OS X setup process on the Mac as if the Mac was brand new. No files, no data, nothing will be on the Mac except for what comes with OS X El Capitan, that is what a clean install is for.
2. If your Mac is a 2011 or later you can use Internet Recovery (command-option-r) to erase the hard drive and reinstall the original OS then upgrade to High Sierra from there. If you insist on using El Capitan you will need to sign into the App Store on a Mac that cannot run High Sierra and download EL Cap, then make a bootable usb installer to use on your MBP.

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Very easy way to reinstall Mac if you are thinking about selling your computer or simply need to install a fresh copy OS X El Capitan follow the tutorial to. Reinstalling macOS is a piece of cake provided you have created a USB install disk for El Capitan to begin with. As you may have guessed, not many people take that extra step. As you may have guessed, not many people take that extra step.

• Reinstall El Capitan on a computer alreadyrunning it

Upgrade from previous versions to OS X 10.11 (El Capitan)

1. From the Apple menu, select App Store....
2. When the App Store opens, search for and download El Capitan. If you don't already have an Apple ID, you'll need to create one. You can monitor the download process in the 'Purchased' section of the App Store.
3. Once the download is complete, the installer should automatically open.

   If it doesn't, or you don't want to complete the installation at this time, you can come back later and double-click Install OS X El Capitan in your Applications folder.

   Within the installer, click Continue, agree to the license agreement, and then choose the disk where you want to install El Capitan.

   After you begin the installation, your computer may need to restart more than once before finishing, and then you may be asked to enter basic information.

4. From the Apple menu, select App Store..., and then install any available updates.

Reinstall El Capitan on a computer already running it

Reinstall while keeping files, applications, and settings

1. Restart your computer, and immediately hold down command-r until the gray Apple logo appears.
2. If prompted, select your main language and then click the arrow.
3. Click Reinstall OS X, and then Continue. Follow the instructions on the screen, and then choose your hard disk. Click Install, and enter your Apple ID information if prompted.
4. When the installation is complete, select App Store... from the Apple menu, and install any available updates.

Reinstall while erasing everything on your computer

El Capitan Os X Download

1. Restart your computer, and immediately hold down command-r until the gray Apple logo appears.

2. If you see a screen asking you to select your main language, choose it, and then click the arrow.
3. Click Disk Utility, and then Continue.
4. In Disk Utility:
   1. Click your hard disk in the left column, and then click the Erase tab.
   2. Next to 'Format:', make sure OS X Extended (Journaled) is selected. In the 'Name:' field, give your disk a name. Click Erase.
   3. Once it's completed, quit Disk Utility.
5. Click Reinstall OS X, and then Continue:
   1. Follow the instructions on the screen, and then choose your hard disk.
   2. Click Install. If prompted, enter your Apple ID information.
   3. Once the installation is complete, you'll be asked to enter basic information, and have the option to transfer data from another computer or backup.
6. From the Apple menu, select App Store, and then install any available updates.

1. Download El Capitan installation from the App store. It might complain that you already have it installed but download it anyway. It will be downloaded to /Applications and be named Install OS X El Capitan.app
2. Create a bootable image from the El Capitan installer on a USB stick larger than 6GB. In a terminal run:

where MyVolume is the name of your USB stick. Change this to match the name of your USB stick.

3. Physically install SSD into the target laptop.
4. Insert the El Capitan USB stick into the target laptop and power up. (This might take a while.)
5. When the menu finally turns up, choose Disk Utility and format the SSD as Mac OS Extended (Journaled) with a unique name.
6. Once complete, exit from Disk Utility and return the main menu.
7. Choose to Install OS X on the formatted SSD.
8. Follow the prompts until El Capitan is installed.

How To Install El Capitan

Reinstall El Capitan Without Losing Data

Here’s link to a video on how to physically install the SSD as well as run the software installation.

How To Reinstall El Capitan Os X

If you’d rather install Yosemite on the SSD then follow the How to Install Yosemite on a New SSD guide.