There are plenty of threads related to El Capitan, yet most of them are irrelevant to the X99 unsupported chipset. Let's attempt here to gather the info we need for all X99 boards...
Installer: We'll create a USB El Cap installer using Apples "createinstallmedia".
1. Verify you have downloaded and have "Install OX X El Capitan.app" in your Application folder.
Format a 16GB+ USB with GUID partition mapping and name it Install. (8GB drives may work for you but I've had a problem with them)...
X99 and El Capitan Install and Misc
Installer: We'll create a USB El Cap installer using Apples "createinstallmedia".
1. Verify you have downloaded and have "Install OX X El Capitan.app" in your Application folder.
Format a 16GB+ USB with GUID partition mapping and name it Install. (8GB drives may work for you but I've had a problem with them)...
X99 and El Capitan Install and Misc