So a few weeks ago I made a new partition to install El Capitan on, thinking that after testing it out I could just erase the default partition (being Macintosh HD - thought it was possible to do until now...). A few minutes ago I just erased and (accidentally) ejected the Macintosh HD partition (it's backed up on external hard drive so no worries to the data) and so now what i'm trying to do is to startup back into Macintosh HD so I can erase the El Capitan partition and download the El Cap OS X into my default Mac HD partition. Problem is, Macintosh HD is not showing on the startup disk panel in System Preferences. So if someone knows how I can go back into my Macintosh HD partition,which is currently not showing on the Startup Disk even after restarting and holding the options key. That way I can erase the El Cap partition, install El Capitan OS X on the Mac HD and then everything will be fine... Specifics: MacBook Pro (Retina, 15 inch, Early 2013) Thanks so much!