Q: Error message "disk encryption failed with the error couldn't unmount disk"

Hi,

My system crashed yesterday and I am still no nearer getting it back up and running.

I have a Macbook Pro Mid 2012, that I cannot repair the permissions on, cannot re-install from a time capsule, cannot re-install OSX, and cannot completely erase and start again.

I am a relative novice but can follow on-screen instructions well enough, but no matter how hard I try to get it to stand up again it just won't play ball.

The message in the title is what I get when trying to completely erase the hard drive. I'd rather not as my latest backup to external drive is a couple of days out of date but if there is no other way, then I'm not fussed. I just need it up and running again.