First I tried upgrading to Yosemite. It hung during the installation, so I rebooted. Now it insists on continuing the installation, and is unable to boot into Snow Leopard that was installed before. The Cmd L log says something about disk permissions, but the Cmd R recovery is unable to repair them. SO, I made a bootable El Cap USB, which boots but DOESN'T give me any option to launch disk utilities. It goes directly to the OS install sequence... and also fails. So I can't repair the disk, can't wipe it, can't update it. What to do?