With OS X El Capitan 10.11 you can create a bootable installer so that you can install using a USB device such as a USB flash drive or external USB hard drive. Just follow these steps…
1) Download the OS X El Capitan 10.11 installer from the Mac App Store but once its on your machine and opens up make sure you DO NOT click install. If you click install then the install will take place and the installer application will automatically be deleted afterwards. So you don’t want that if you want to have the installer on a handy USB device, so just download it but don’t install.
2) Insert your USB device and use Disk Utility to format it (this will obviously erase everything on the USB device so make sure you’ve copied anything you need off it beforehand). Name the drive ‘OSXInstall’ (without the quotes). Note that the installer needs at least 7GB free space so your USB device will need to be at least that size.
3) Open Terminal.app and run the following command…
sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/OSXInstall --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app --nointeraction
Because you are using sudo you will be asked for your password before the command starts running so make sure you type in your password and press return to start the process.
4) You’ll see some output in Terminal letting you know it’s copying files. The bit that takes the longest is the one that says ‘Copying installer files to disk’ and it can take 10 minutes or longer depending on the speed of your USB device so be prepared for a bit of a wait at that point.
When that’s done your USB will be ready with the bootable installer. It will also have been automatically renamed to ‘Install OS X El Capitan’. You can now use it as a bootable USB installer on any Mac.