OSX external backup disk cannot be mounted.

Tried Disk Utility, it cannot fix the disk.

I then let default fsck_hfs run to fix it by default, based on,
http://apple.stackexchange.com/questions/130304/how-can-i-repair-a-disk-that-disk-utility-says-it-cant-repair

'top' shows that 'fsck_hfs' was running when "Horizon green backup disk" was connected to "byte".

Byte-4:~ hqin$ ls /dev/disk*

/dev/disk0 /dev/disk0s2 /dev/disk1 /dev/disk1s2

/dev/disk0s1 /dev/disk0s3 /dev/disk1s1

Byte-4:~ hqin$ ps -ef | grep fsck

    0   623    16   0 12:31AM ??        21:22.89 /System/Library/Filesystems/hfs.fs/Contents/Resources/./fsck_hfs -y /dev/disk1s2

  502   741   451   0  1:07AM ttys001    0:00.00 grep fsck

Byte-4:~ hqin$ df -hl

Filesystem                          Size   Used  Avail Capacity   iused    ifree %iused  Mounted on

/dev/disk0s2                       698Gi  506Gi  191Gi    73% 132783864 50149878   73%   /

localhost:/bCr671Ajn4MZTpa3yc_-X4  698Gi  698Gi    0Bi   100%         0        0  100%   /Volumes/MobileBackups

In the evening, the disk was finally fixed and mounted.