Erasing External Harddisk that cannot be opened

I got problem with my external hard disk. After some issues of umounting the disk, I couldn’t open the hard disk anymore. I tried to use Disk Util to repair, but it said it cannot be repaired and must be erased. But I can’t even erased it! It says disk can’t be opened. So I finally managed to erase it.

First, go to Terminal and type:

$ diskutil list

This will show a list of disk and partition. It happens that I can’t seem to erase the volume. But somehow I can erase the disk. So this what I do.

$ diskutil unmountDisk force /dev/disk1

and after it unmount, I type:

$ diskutil eraseDisk JHFS+ Backup /dev/disk1
Started erase on disk1
Unmounting disk
Creating the partition map
Waiting for the disks to reappear
Formatting disk1s2 as Mac OS Extended (Journaled) with name Backup
Initialized /dev/rdisk1s2 as a 931 GB case-insensitive HFS Plus volume with a 81920k journal
Mounting disk
Finished erase on disk1