Because of this, I recommend creating your own bootable El Capitan (OS X 10.11) installer drive on an external hard drive or USB thumb drive. If you need to install El Capitan on multiple Macs. The installer you create can be used to install OS X El Capitan 10.11 on any eligible Mac computer. This is a more convent and less time consuming way to install OS X El Capitan on multiple machines, gives you a handy emergency disk, and enables a fresh OS X install. Using the DMG download of OSX 10.11 El Capitan (see above) and making a bootable USB-installer with Diskmaker X I finally managed to install 10.11 without installing 10.6.8 Snow Leopard first! I don't know if I was refused to do this before because I had either tried to install it on a partition on a drive or because I named the partition prior.

Install Os El Capitan From Usb File

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.

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

Os El Capitan Download

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