Using Disk Utility to Erase Disk-won't unmount

excellent advice desbrisay when combined with what niccodeamus suggested. Was about ready to thromw WD My Book Studio through the window until I found this (My Book failed to unmount when erasing or partitioning)

Thanks so much this worked!!!

I had similar problem, can't unmount easily.

Ending up connecting the disk as a target disk (holding T key on startup) connected to another Mac.

Then using terminal, used the following commands to force unmount disk and erase the disk after.

To check for the exact name of the disk:

sudo diskutil list

(you will see the list of drives/partitions, take note of which one is the one you are trying to unmount/format, should be something like /dev/disk<number>)

Then to force unmount disk:

sudo diskutil unmountDisk force /dev/disk<number>

You should get a response to the success or failure to the unmount command.

Then if you want to format force format the disk:

sudo diskutil erasedisk JHFS+ Untitled /dev/disk<number>

I tried turning off indexing via terminal unsuccessfully but eplt@odw's Force Unmount worked perfectly.

Background:

I boot my imac from an external SSD and use the original, internal hard disk to clone a backup to. My partitions needed resizing so I had to erase and repartition. During this process the disk wouldn't unmount,

All good now though. Thanks for your post eplt@odw

ps -ax | grep fsck

and then killed that process:

sudo kill <process number>

That solved the problem.

Thanks.

I've tried all of these suggestions and nothing is working. Still saying can't unmount disk - at a dead end, any ideas?

I had a volume named W for Western Digital etc... wanted to erase it to give to my father to use as his Time Machine backup. It wouldn't unmount directly when trying to 0 out data, it was because I had Quicktime open and had been playing a video stored on the HD going to be erased. I closed the Quicktime app and directly unmounted the volume, then highlighted the drive once again and it erased the data as desired. Sometimes things don't go so smoothly but it can be a simple oversight sometimes as well.

Great thread, I was about to burn my external harddrive!

Worked perfectly!

THANKS!

Another option for anyone having to deal with an internal drive that will not erase or partition, giving the message that the erase failed because it "Could not open Device"

This happens to me when I try to wipe and install a fresh OS on any internal HD using a (Mavericks) installer on a bootable external drive.

Try connecting the Computer in question to a functional Mac with a firewire cable. Boot up the faulty computer in Target Disk Mode (hold down the "T" key) and it should show up on the functional Mac with the message that the disk is unreadable. Go to Disk Utility and it will now allow you to erase and then partition with 1 GUID partition. (Make sure you are erasing the correct drive !!)

Now unmount the target computer, shut it down and then restart holding down the option key to select the bootable external from which you can now re-install the new OS!

Works each time for me!

PS

THE EASIEST WAY TO UNMOUNT:

Simply use Disk Utility to "Force Unmount". You do this by selecting the partition in the left pane, and OPTION+CLICKING the "Unmount" button. Once you have done this, you can format the entire Volume.

I hope this helps someone else, as I struggled with this for a few hours. I then remembered that the OPTION key on Mac OS is a frequent ticket to awesome features.

I just had this problem and tried everything.

Here is what finally worked - I connected the external drive to a PC and did a quick reformat, then plugged it back into my mac and was finally able to erase it and format it in Mac OS Extended.

Thanks niccodemus! Terminal option to temporarily disable Spotlight was just what I needed; worked like a charm.

Don't know if this works out, in my case it did, and it is quite simpler for people not customed to use terminal, or people uneasy with using sudo, or simply, like me, people who don't have a password in their daily user account:

my solution to "can't unmount" was to put the disk in Spotlight of the accessing Mac into the Private Sphere of Spotlight preferences, so the background indexing did not avoid the unmount. Worked out with a MBP in target mode.

Put if that fails, for sure the terminal will sort it out 🙂

http://www.mactrast.com/2012/07/quick-tip-force-a-disk-to-unmount-in-os-x/

"The easiest and safest way to force a volume to eject is to open a new Finder window, locate the drive or volume in the left-hand column, right-click or option-click the volume, and select Eject “[Volume Name]” in the drop down menu."