Force Repair/Unmount External Hard Drive? I was backing up my Mac today (Time Machine) when I think I jostled the USB cable.  The hard drive disappeared from the finder window and progress on backing up in the Time Machine window disappeared.

I've tried unplugging/replugging the HD and restarting the computer, but it doesn't show up in finder or Time Machine anymore.  The light on the hard drive is blinking, indicating it thinks its being accessed, but I don't think it's actually doing anything.

The HD DOES show up in the Disk Utility Program.  However if I try to repair the Time Machine partition, it says "Disk Utility can’t repair this disk. Back up as many of your files as possible, reformat the disk, and restore your backed-up files."  If I attempt to erase the partition, it says "Volume Erase failed with the error:  Couldn’t open disk."  And if I attempt to erase the Disk (one level up from the partition in the Disk Utility), it says "Disk Erase failed with the error: Couldn’t unmount disk."

I tried force unmounting the disk using the Terminal command, but the disk doesn't actually show up in /Volumes/, so it didn't work.

Any ideas on this one?  Is the disk completely totalled or is there another way to get it to reset so I can re-back things up?
You can use the device id as well as the mount path for diskutil. Get the device id using diskutil list (disk3 in my example) and then force unmount it:

diskutil unmountDisk force disk3


And then erase the disk (HFS+ with GUID Partitioning Table):

diskutil eraseDisk HFS+ Untitled GPT disk3