Can't erase unmountable external HD My external HD isn't mounted and can't be repaired by Disk Utility. However, I'm also unable erase the disk:

$ diskutil list disk2
/dev/disk2
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *4.0 TB     disk2
   1:                        EFI EFI                     314.6 MB   disk2s1
   2:                  Apple_HFS Everest                 4.0 TB     disk2s2

$ diskutil repairVolume /dev/disk2s2
Started file system repair on disk2s2 Everest
Updating boot support partitions for the volume as required
Error: -69673: Unable to unmount volume for repair

$ diskutil eraseDisk JHFS+ Everest /dev/disk2
Started erase on disk2
Unmounting disk
Error: -69888: Couldn't unmount disk
You can try entering in Single User Mode and do an fsck on the hard drive. 

To enter single user mode, turn on your computer and immediately after press the Command s. This will load you to a terminal looking screen. Let it run its pre-checks. 

After it is done there should be two commands to run:


a command to mount the drive 
a command to run fsck on the drive. 


Do the fsck first. This will make sure that at least the hierarchy of the OS file system is ok. Use the mount command just to see if you can mount the Drive. 

If it fails, there could be a hardware issue, a cable or failing hard drive. If it does not fail and passes everything, reboot. See if it works normally.

I believe the following command is for the fsck

/sbin/fsck -fy

and the following for mounting

/sbin/mount -uw /

Single user mode will display the correct commands.

Once you are done with Single User mode type reboot and hit Return

And this will boot you up normally.