I am currently running High Sierra but require El Capitan for old software. Reformatted an old external hard drive and created a bootable installer USB. Restarted the computer opening the installer and indicated to install El Capitan on external harddrive. Installer finished and restarted. Upon reboot (holding down option) the external hard drive is not listed as an option. The external drive shows that it only has 300mb on it which does not seem right for a full install of El Capitan. Any suggestions would be appreciated.