Using Disk Utility to Erase Disk-won't unmount

I realise that you have solved this problem, but I have solved a similar issue differently.

It appeared that the external drive would not unmount because some activity was using it. fsck_hfs process was working away as shown by activity monitor. It seems that Spotlight was struggling with a the failed drive and would not let it go so the unmount and erase could not occur.

This was solved by disabling Spotlight (terminal: sudo mdutil -a -i off) temporarily. After disabling Spotlight, Disk Utility performed its job and my corrupted drive was restored to use.

(Spotlight is turned back on with: sudo mdutil -a -i on)

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."

Thanks for the above info!

I had a similar problem, Disk Utility would not erase and reformat my external disc and complained about being unable to erase the external disc.

I tried disabling Spotlight as described above, but that didn't fix the problem.

I then searched for an fsck process:

ps -ax | grep fsck

and then killed that process:

sudo kill <process number>

Problem solved! Disk Utility then successfully erased and reformatted the external drive!

Many thanks for the clues that led me to this solution!

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

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.

Had a similar issue, The drive would not unmount in disc utility to be erased, disk warrior could not even see it.

I used the sudo command that niccodeamus suggested, I also forced quit the fsck_hfs process, my drive then began to erase without issue

Thanks 🙂

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)

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.

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

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.

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 🙂

That did it for me, thanks niccodeamus.

I have a FreeAgent Goflex Seagate 3Tb Drive with thunderbolt cable which suddenly stopped working, Disk Utility was able to see it but could not unmount it resulting in not being able to repair or format disk. Here's the solution.

1- sudo mdutil -a -i off

2- Force Quit process fsck_hdfs from the activity Monitor

3- Immediately Erase Disk in Disk Utility (before fsck_hdfs starts again).

That's why the disk couldn't be unmounted because it was in use by spotlight indexing for some weird reason.

thanks nicco this helped and I am not sure if it was your command or going into the activity monitor and force quitting the fcsk thingy that starts when the disk utility fails as I tried a combo and both together allowed the format to happen without leaving the OSX environment.

After a long time on forums trying to recover a WD external drive I need to re-***** my backup strategy. I could imagine many non or less savy people thinking their files are lost and worse getting charged by people and still not getting their precious files back. I guess the cloud fits these people best - but really large backups of things over the cloud just seems too big to be handled.

Somehow i missed the tip about running ps ax |grep fsck when looking earlier.

But after the various permutations of diskutil unmountDisk failed, i finally got to the point of realizing that fsck_hfs was permanently running.

Another option is pkill:

sudo pkill fsck_hfs

(It's better to run ps ax and make sure you are killing the right one, but ...)

Just because it took understanding what was going on to click: When one plugs in an external drive, fsck_hfs is run on that drive.

With some special drives in a bad state, fsck_hfs loops over and over trying to fix the drive.

This leaves the drive unmounted, but also in use and not ejectable, writable, dd-able, etc.

The annoying part is that you can run fsck_hfs on the /dev/rdisk# more than once at the same time. So Disk Utility will try to repair the drive. Even if you boot into Recovery Mode, this happens. And you can try to run fsck_hfs there and it will run.

But, if run with the right parameters, it will be stuck in the same loop. (Which i did, which helped me realize how the disk was stuck in the state it was.)