I was backing up with time machine to my 1tb wd external hard drive and it said the backup failed for some reason. So I go to disk repair and try to repair the disk. it says "error couldn't unmount disk" I don't know what to do at this point, nothing I have tried has worked. I tried fixing it from pressing cmd r on startup, same problem. Nothing seems to work. i tried to delete the partion, with the same error. Is the HDD dead?

Im running mountain lion right now, my mbp specs are to the left below my name