Trying to erase a hard drive in a MacBook Pro so that I can re-install Yosemite but receive a "Volume Erase Failed" and "Couldn't Unmount Disk." I am booting from a USB drive with Yosemite on it, not from the internal hard drive. I also took the hard drive out, connected it to my iMac, and tried to erase it that way. I don't get the same message, but the erase does seem to take abnormally long and get stuck half way. Then it seems to finish but the disk never gets renamed. Is the hard drive finished and do I need to buy a new one?