osx capitain install failure now cannot restore from backup. When trying to select a destination I cannot select the internal hard drive as it cannot unmount

I have an old MacBook which was running fine on OS 10x. Tried to upgrade to Capitan but it has failed. Now when I try to restore from mybackup I get to Select a Destination I cannot see the HDD. I've searched other forums and they suggest unmounting but whenever I try unmounting in the terminal I get errors saying the resource is busy.

Can someone please help?