I have been trying to do a clean install of El Capitan on a friend's MacBook Pro late 2011. He has already upgraded but he is curious to see how the process goes with speech as this is his first time on Mac. We started by Command+R and Power Key and waited for sometime until the first screen appears. I remember there were five options, one is the Disk Utility. We chose this and moved to the next page to erase data on the internal HD. The usual table of HD's attached to the Mac appear and we selected the one El Capitan is installed on. There has been no erase button, just a page with details about the selected HD. In the Toolbar we found the Erase button and by selecting this we got the message that it can not be erased. I googled for that and found that there are those who faced the same problem and there has been no solution except making sure that there is nothing running while trying to do the clean installation. Could it be VO that prevents us from initialising the installation?? To make sure that we were not doing it the wrong way, I tried with my own Mac Air 2015 and that was different as the erase button in my case was already dimmed. Again, I googled for this and the same recommendation was there to insist on having no processes are to be running while trying to wipe data off on HD. Anyone has suggestion, please?