Unable to erase hard drive Macbook pro mid 2009
Hello – I can't get help from Apple support for this so I'm appealing to the experts.
Macbook Pro 15" mid 2009. The machine had El Capitan installed and was working fine until suddenly it refused to boot up. I erased the disk and installed Snow Leopard using a CD. It worked. Then I had the brilliant idea to try to upgrade to El Capitan. I downloaded it from the Apple website, opened from within Applications and went through the install process. It got to 1 second remaining and then it claimed that it could go no further. In successive attempts, it provided a helpful log full of report crash etc. information. (I've reinstalled El Capitan on this machine in the past and put in the terminal command to change the date so the OS would work but none of this knowledge has helped this time around. Terminal doesn't even accept that the date could be changed.)
I then tried to erase the hard drive but Disk Utility said it could not complete the task because it could not unmount the disk. In addition to the hard drive, there are 2 disk images showing called OS X Install ESD and OS X Base System. I can't do anything with either of those. I can run first aid on the hard drive and though it says it's been repaired, I still can't erase it.
I can't get the machine out of this loop of trying to install El Capitan. I would like to use my Snow Leopard CD and just go back to that operating system but I am not sure when to put in the CD to allow it to be recognized rather than El Capitan. There must be a terminal command to erase the drive and/or make this loop stop. Or perhaps there's a keyboard command and a time to put in the CD that would work?
Thank you for any guidance. Rachel
MacBook Pro, OS X 10.11