I did a bad mistake: my Macbook pro was running very slow with yosemite, and I've tried to reinstall factory OS, but after applying option+cmd r, I decided to erase the deleted space in the main drive (not the "Macintosh HD" partition) in Disk Utilities, and, because it was taking ages, I interrupted it in the middle. Now the "Macintosh HD" is named "Untitled" and is unmounted. When I try to erase it (to make it mount again), it says "can't find mount point". A mess. Can anyone help me to recover the drive or mount this "Untitled" partition, in order to reinstall the OS from scratch? Is there any Terminal codes to clean this problem?