Can't unmount Macintosh hd with Mavericks with lion bootable USB

Hi I'm trying to revert back to lion after using the sixty four on thirty two method to install Mavericks on a mac pro 1,1 but for some reason when I boot from a USb with the lion installer I can't erase the Macintosh hd it says it can't unmount so I was wondering if there was something I had to do specifically. Also I can't run command+r to start on recovery. Hoping you can point me in the right direction.