The following is taken from my El Capitan quick start. For more info, click the link given for the book. The script discussed is not included in this guide. For info on the script click the book's landing page above.
creating a boot thumb drive for your brand new Osx El Capitan Installer
The following is modified from this link.
Suppose you want to install from scratch and your mac's hard drive for some reason won't let yo do it via the recovery console. Suppose you just want to reformat your mac and start over, or sell it. This final chapter will walk you through creating a bootable thumb drive that will help you do just that.
What you will need
You will need a flash drive that is at least 8 gigs or higher. You will also need about 15-30 minutes on your hands to wait for the process to complete. You might also need a cup of coffee while you wait. Joking of corse.
First, you will need to if you did not already download the osx 10.11 installer from the app store. This is about a 6 gig download. See the upgrade chapter on how to do this. A side note to the reader here. If you have any important content on the flash drive, move it somewhere else. The drive will be erased of all of its content.
Next connect your USB flash drive to your mac. If you are using a different medium such as an external drive, connect that.
Next hit command plus shift plus c from the finder and hit enter on your new connected drive. After this, name it untitled. This is easier to type, and has no spaces. See the next step.
Now it comes time to launch the terminal. don't worry you don't have to know 100 percent what you are doing. This is going to be a simple copy and paste command.
- Launch the terminal. Type Command Shift u from the finder, navigate to the terminal and either click, or type command down arrow to open the app.
- paste in the following command. Remember we renamed the drive "untitled"
- sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/untitled --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app –nointeraction
- Hit enter, then enter in your admin password. The sudo command tells the computer you are temporarily God and it is doing things your administrator account is not normally aloud or in most cases able to do.
- Hit after you enter in your password, hit enter. and wait. Now would be the time to grab that cup of coffee and finish it in about 15 minutes.
When your prompt comes back, that is, when you see for example Sarah's' mac or what ever you saw before pasting in the command you are done. Feel free to rename the flash media to something useful, but remember to rename it untitled if you ever want to update your boot disk. for the convenience of the reader I've included a script with this guide. You can download the script and paste it in to your home directory, launch the terminal and simply type a period, then a slash followed by the script name. Remember to include the extension at the end or the command will die a violent death. and you will need to reenter the command again. You will be asked for your admin password once you type in the command and hit enter. just as if you typed in the command from above. Note should the script fail type the following in to the terminal.
chmod 755 ./updateos10.11.sh
Then press enter. After that try the script again. You will get your password prompt after which the script should work.
The above steps might seem daunting to the beginner but I promise that these steps are as easy as pie, or what ever you want to have while you wait for this process to complete. This is my 100 percent promise.