So I plan to erase my MacBook Pro completely. After I erase its drive from an El Capitan recovery drive that I have, I want to boot into Internet recovery mode, plug in another flashdrive, install Mavericks on it, and then use the recovery drive for El Capitan to install El Capitan onto my MacBooks internal SSD.
Basically, I want to have El Capitan on my internal SSD and Mavericks on my external USB drive. Is this possible at all, to have an older version of OS X installed on an external device?