mac wont boot up out of recovery mode and wont let me erase start up disk w every terminal comand!

That is the problem, I cant restart out of recovery mode. Every time I go to restart, it wont see any disk to start up from, nothing appears in that window and when I shut down and restart , it starts back up in recovery mode without pressing any key.

Starts up with 4 choices, the choice of re install OS wont work- says disk is locked, and cant erase from the disk utilities, which is ultimately what I want to do.

With several terminal commands, I found on the internet, I cant do that either, since the disk wont "unmount" and allow me to erase in disk utility window.

Its my friends Imac mid 2011 on Yosemite, with a hard drive thats almost full. I have backed it all up successfully and she wants me to erase it and help her sell it.

Thanks so much for responding!

Ok ! I will try that suggestion. thanks for responding

It is not solved yet actually, but I will try your suggestion now. If I am unsuccessful, I will need to post agin, I imagine.thanks again