Can't unmount internal harddrive

Earlier today upon starting up I would get lines of code on my screen then the screen would turn black. I tried to erase the harddisk to start fresh but I can't repair the hard drive or erase it since it cant unmount.

I have read other posts with similar problems but these seem to be external harddrives.

I have reset my PRAM but nothing has changed

It may be nothing but when i try to start the mac in recovery mode to acess the harddisk I always see the apple logo first and it tries to boot up normally before switching to the disk utlities menu.

I think it may be something to do with the Base OS being active as i try to erase the Hard disk but i cant do anything about it form recovery mode