Are you trying to make a bootable installer? Do you have the El Capitan installer downloaded from the App Store? You cannot make a bootable DVD but you can make a bootable flash drive.

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1. To install El Capitan, make sure the USB drive is inserted into the computer, then restart the computer while holding the Option (alt) key on your keyboard. Wait until the Startup Manager pops up, then select the 'OS X Base System' volume. Alternatively, you can just hold down the C key while starting up to boot directly into the USB drive.
2. I want to create a bootable El Capitan external drive (and retain, for the time being, my older OS on my internal hard disk). I partitioned an external drive using Disk Utility. I went to the App Store to download El Cap but stopped the process as I don't want it to overwrite my current OS as i need both OS to run some older applications.

Bootable USB Installers for OS X Mavericks, Yosemite, El Capitan, and Sierra

Oct 01, 2015 OS X El Capitan is finally here, and if you're looking to install Apple's new desktop OS fresh from the ground up, then you'll be needing a bootable USB flash drive to get started with the process.

First, review this introductory article: Create a bootable installer for macOS. Second, see this How To outline for creating a bootable El Capitan installer. Simply replace the Terminal command with the one from the preceding article by copying it into the Terminal window. You will need an 8GB or larger USB flash drive that has been partitioned and formatted for use with OS X.

Drive Partition and Format

1. Open Disk Utility in the Utilities' folder.
2. After Disk Utility loads select the drive (out-dented entry with the mfg.'s ID and size) from the side list.
3. Click on the Erase tab in the Disk Utility toolbar. Name the drive, 'MyVolume'. <---- IMPORTANT!
4. In the drop down panel set the partition scheme to GUID. Set the Format type to Mac OS Extended (Journaled.)
5. Click on the Apply button and wait for the Done button to activate. When it does click on it.
6. Quit Disk Utility.

Create Installer

Open the Terminal in the Utilities' folder. Choose the appropriate command line (in red) depending upon what OS X installer you want. Paste that entire command line from below at the Terminal's prompt:

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Command for macOS High Sierra:

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sudo /Applications/Install macOS High Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install macOS High Sierra.app

Command for macOS Sierra:

sudo /Applications/Install macOS Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install macOS Sierra.app

Command for El Capitan:

sudo /Applications/Install OS X El Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install OS X El Capitan.app

Command for Yosemite:

sudo /Applications/Install OS X Yosemite.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install OS X Yosemite.app

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Command for Mavericks:

sudo /Applications/Install OS X Mavericks.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install OS X Mavericks.app

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Press RETURN. You will be asked for your admin password. It will not echo to the Terminal window. Then press RETURN again. Wait for the return of the Terminal prompt signifying the process has completed. It takes quite some time to finish. Be patient.

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