Fix for disk erase failed couldn’t unmount disk

If you’re trying on a Mac using Disk Utility to erase a hard drive and it won’t unmount, giving you an error similar to Disk erase failed. Couldn’t unmount disk, then you may have to force an unmount through the terminal.

Launch up Terminal.app (through /Applications/Utilities or through a Spotlight search).

Then paste in the command:

diskutil list

to list out the different disks. You should see some disks appear like /dev/disk0, /dev/disk1, /dev/disk2.

Find the one you want to force unmount. For this example, let’s say it’s /dev/disk2.

Run a command similar to this one

sudo diskutil unmountDisk force /dev/disk2

If you’re prompted for a password, enter it (yes, your account does need to have administrative privileges). You should not physically unplug the drive.

You should now be able to erase the drive.

Posted

April 13, 2015

Uncategorized

alanysiu

Tags:

disk utility, diskutil, erase drive, mac, unmount

Comments

75 responses to “Fix for disk erase failed couldn’t unmount disk”

Randall

July 14, 2015

This worked perfectly for me! After searching for a very long time through forums of people suggesting basic things or trying to make others feel ignorant this solution worked!
Thank you so much. I now have a useable disk where before it would not mount and could not be formatted or erased. Running OS X 10.9.5. I will be linking this solution to other forums in the future.
http://technology.siprep.org/fix-for-disk-erase-failed-couldnt-unmount-disk/

Reply
Bobby

October 9, 2015

Excellent post! It worked the 1st time for me, and i also had to stumble across the interweb minefield before finding this solution. Thank you !

-Bobby

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Lissett

October 11, 2015

Worked for me as well! Thank you so much 🙂 Very clear instructions!

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Jason

November 5, 2015

I tried this method, but now it says “Forced unmount of disk2 failed: at least one volume could not be unmounted” Please help, I would like to erase my hard drive so i can do a clean install of el Capitan

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Alan Siu

November 5, 2015

Odd. I’ve never seen that. I did Google your error, and came across this workaround. Give it a shot:
http://emsooriyabandara.blogspot.com/2014/11/solved-external-hard-drive-not-detected.html

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Jaycie Turner

December 11, 2015

Whenever I go to type the password, it has a round dot and won’t let me type anything. Do you know what might be causing this?

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Alan Siu

December 11, 2015

In the terminal, there’s no visual feedback for passwords. Just keep typing your password and then hit Enter when you’re done. Your password is being accepted. There are just no dots or asterisks to indicate so.

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marita

January 26, 2016

What to do when the problem happens when i am trying to erase the internal hard drive to sell my pac book pro?

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1. Alan Siu
  
  January 26, 2016
  
  Try booting into recovery mode (holding down Cmd-R during bootup) and then erasing it from there.
  
  Reply
Ian Small

January 30, 2016

Brilliant! Spent hours searching. Came across this and bingo, worked in seconds.

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Scott

February 12, 2016

Great tip. This worked great. Took a lot of time to find it but was a flawless solution. Thanks very much.

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Jeff Alexander

April 15, 2016

I have been trying off and on for weeks to get rid of this error. I searched high and low and your directions were the easiest to follow and actually worked!! Thank you very very much.

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Jon

April 18, 2016

When I try to do this I get an errror on the terminal saying Sudo command unknown… Any ideas?

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Alan Siu

April 18, 2016

That’s an odd one.

What happens if you try this?

sudo /usr/sbin/diskutil unmountDisk force /dev/disk2

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1. Ashley
  
  June 25, 2016
  
  Came across this same problem after my MacBook Pro freezes after logging in and won’t go to my desktop. I’ve tried the original text and the one you suggested to Jon and it still says ‘command not found’
  Any ideas what’s up?
  
  Reply
  1. Alan Siu
    
    June 26, 2016
    
    The only thing I can think of is that you’re accidentally typing it wrong. I would suggest copy and paste. You may have to still change the number (it may not be /dev/disk2 but /dev/disk3 or something else), but most terminal “command not found” errors I’ve seen have been due to people not typing exactly what they’re seeing (no space where there should be one, or an I instead of 1, or lowercase instead of uppercase, etc.), so copy-and-paste is probably the best way to go.
    
    Reply
    1. jmbice
      
      January 24, 2017
      
      I am getting the same error as Jon and Ashley. Can assure you I am typing it in correctly…
      
      Reply
      1. Alan Siu
        
        January 25, 2017
        
        Another possibility is that you’re trying to run the sudo command from a non-admin account. Standard accounts do not have enough privileges to escalate using sudo.
joe

May 4, 2016

Thank you! Worked perfectly after many failed “solutions”.

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Ava

July 8, 2016

I was trying to install tacos sierra on my 2011 iMac 21.5″ and it crashed my computer, and it just kept restarting. I foolishly tried to download it on my macbook pro 2012 13″ also. So now i have two broken internal hard drives. Have an external hard drive, which i used to boot up the iMac with el capitan, but i am still in the process of trying to erase the internal drive. I tried this but it still didn’t work, it says “unable to unmount 1 or more part” or something like that. I need to get this one fixed and get the internal one running again, because i can’t have two computers running off one external drive, and i still need to fix my macbook. PLEASE HELP!! Thank You!

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1. Alan Siu
  
  July 8, 2016
  
  Sierra is still beta, right? I wouldn’t have installed it on an everyday use (production) machine, but I guess what’s done is done. If the external El Capitan absolutely refuses to unmount the disk, can you try booting into Recovery Mode (hold down Cmd-R while booting up) and then using Disk Utility there?
  
  If that doesn’t work, I’d actually seriously consider booting Ubuntu or some other non–Mac OS X operating system to reformat the drive that way first, and then use Disk Utility to format the drive back to HFS+ and reinstall OS X.
  
  Reply
  1. Ava
    
    July 8, 2016
    
    i managed to install el capitan on the external drive and use terminal to force erase the disk. from there i booted the iMac on its original OS X lion and from there downloaded the public beta of mac os sierra, which i was supposed to in the first place, but i was just impatient. Im just happy i didn’t have any files on the computers, so nothing was lost, just my time it took to fix them. Right now i have the macbook booted on the external on el capitan and I’m going to go through the whole process over again for this computer.
    
    Reply
Nicola

July 26, 2016

Thank you!! So much easier and clearer than anything else I read. Worked immediately.

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Lauri

August 17, 2016

I had hoped…. 🙁

Didn’t work for me – I’ve been trying anything and everything to get my external to show back up so I could start over!

Unmounting disk
Wiping volume data to prevent future accidental probing failed.
Operation failed…

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1. Alan Siu
  
  August 17, 2016
  
  Based on the output you posted, your disk might have unmounted just fine. It looks as if the failure came from the wiping part. You may find these links helpful:
  https://www.ifixit.com/Answers/View/244571/partition+failed+wiping+volume+data+to+prevent+future+accidental+probi
  https://www.reddit.com/r/applehelp/comments/3v3ybd/wiping_data_to_prevent_future_accidental_probing/
  
  Reply
brian

August 26, 2016

Awesome! thank you so much.

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Dr1ver105

September 4, 2016

I knew there had to be an “easy” way to do this, and this was it. Thanks so much for the quick and easy tip. Worked like a charm for me to reformat an external drive. 100x easier than the other ways I read to solve the unmount issue, like boot disks, etc.

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Ben

September 17, 2016

Is there a way to work around needing a password? I was an idiot and renamed my home folder and now I’m trying to format my computer because it doesn’t recognize me or my password so not only can I not get into any admin stuff to fix the original issue, but I get the cannot erase/unmount error, and my password doesn’t work so I can’t use the terminal to force it. u_u

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1. Alan Siu
  
  September 17, 2016
  
  You may be able to rectify this issue by booting into single user mode, and then (after the fsck scan) renaming the home folder back.
  
  Reply
Shoug

September 20, 2016

i have the same prob and am doing this coz i forget ma macbook password and now !! i can’t complete this w/o a password !! wt to do pls i need help ..

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1. Alan Siu
  
  September 20, 2016
  
  You really have to tackle one problem at a time. First, figure out your password. If you can’t remember it, reset it (you can do so from recovery mode):
  https://discussions.apple.com/docs/DOC-4101
  
  Reply
Jerry

October 5, 2016

thx

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Nate

October 25, 2016

when i try to unmount i get an error saying “at least one volume could not be unmounted”. I tried the other workaround; hdiutil detach, and i get an error saying “Resource busy”

please advise

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1. Alan Siu
  
  October 25, 2016
  
  This should help you figure out what it’s busy with, so you can manually kill that process:
  http://apple.stackexchange.com/a/104850
  
  Reply
Leslie

October 26, 2016

Your instructions are clear, and almost successful in my case…
Within the terminal it was successful, but when I go back to Disk Utility to format the hard drive to ExFat it still says “Disk Erase failed with error: Invalid Request.” Any ideas?
(I am trying to format a windows pre-formatted hard drive to use on my mac).

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1. Alan Siu
  
  October 27, 2016
  
  This may help you:
  https://youtu.be/u6dnYe7TDMQ?t=82
  
  Reply
Kristie

November 15, 2016

Thanks so much, great fix!

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chris

December 1, 2016

I’ve done this procedure several times, it says it was successful: “Forced unmount of all volumes on disk2 was successful” but it’s not showing up in finder, and the disc utility says it’s uninitialized, still fails disc utility erase, and after hitting the erase button it tries, but fails: Unmounting disk
Operation failed…
Unable to write to the last block of the device.

I watched the video posted here that says to partition it, but my partition button is greyed out, what can I try next?

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1. Alan Siu
  
  December 5, 2016
  
  If it successfully unmounts the volumes, the volumes are unmounted and thus should not show up in Finder.
  
  Keep it unmounted and then erase in Disk Utility.
  
  Reply
  1. chris
    
    December 8, 2016
    
    it still fails disc erase in utility, this drive is almost brand new, it’s hardly been used, is there anything else I can try?
    
    Reply
    1. Alan Siu
      
      December 8, 2016
      
      Try booting up Ubuntu and reformatting the drive (as FAT32) and then booting into Recovery Mode and re-reformatting it (as HFS+).
      
      Reply
Del Rey

December 3, 2016

Worked for me reformatting a Seagate Backup plus to use with OS 10.6.8. Thank you!

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Toby

January 5, 2017

Is there a way to restore the data prior to erasing using the steps above?
Current error: Source volume is read-write and cannot be uncounted, so it can’t be lock copied.
Thanks.

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Toby

January 5, 2017

Current error: Source volume is read-write and cannot be unmounted, so it can’t be lock copied.

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1. Alan Siu
  
  January 5, 2017
  
  The error says “so it can’t be lock copied”? I’ve never seen that error and can’t find it in a Google search. What’s the exact error message? And what are you trying to do? Get data off the volume before erasing it? I’m unclear what you mean by “restore the data.” Do you mean “save the data”?
  
  Reply
Sophia

January 11, 2017

That one did the trick. Perfect solution, thank you so much.

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If a /dev/diskX can’t be force unmounted… – St. Ignatius College Prep Tech Blog

January 25, 2017

[…] a sequel to Fix for disk erase failed couldn’t unmount disk, if you really cannot even force unmount a disk, chances are it's being used by some process. I […]

Reply
mauop

February 17, 2017

Great piece of advice! Worked at first attempt, I was running out of hope after trying several solutions from other forums

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Christopher

March 7, 2017

Thanks for the post. I’ve tried everything you recommended (including in the comments section), and have not had success. Here’s what I did:

Big-Mac:~ admin$ diskutil list
/dev/disk0
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *500.1 GB disk0
1: EFI EFI 209.7 MB disk0s1
2: Apple_HFS iMac 499.2 GB disk0s2
3: Apple_Boot Recovery HD 650.0 MB disk0s3
/dev/disk2
#: TYPE NAME SIZE IDENTIFIER
0: FDisk_partition_scheme *1.0 TB disk2
1: Apple_HFS Beatrice 910.3 GB disk2s1
2: Windows_NTFS For Windows 89.9 GB disk2s2
Big-Mac:~ admin$ sudo diskutil unmountDisk force /dev/disk2
Password:
Forced unmount of disk2 failed: at least one volume could not be unmounted
Big-Mac:~ admin$ hdiutil detach /dev/disk2
hdiutil: detach: timeout for DiskArbitration expired
Big-Mac:~ admin$

Any help would be appreciated! 🙂

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1. Alan Siu
  
  March 7, 2017
  
  Yeah, unfortunately, no idea on that. Honestly, since that appears to be an external drive, I would shut down the computer and then just unplug the drive.
  
  Reply
Nathan s

March 20, 2017

I have tried all the methods listed in previous comments. When trying to erase the disk it gives me the “could not unmount disk” error. When using the sudo command it gives me the error of “at least one volumes could not be unmounted”. I used to have a boot camp windows 7 that was deleted a year ago If that would effect it. I am not using any external drives

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Al

April 12, 2017

After trying for several hours, came across this solution and it worked perfectly, thanks much for posting this! I am wondering now if I could have simply “ejected” my external Seagate hard drive to come up with the same solution – since drive disappeared from desktop similarly when I choose to eject drive. Either way, glad this worked.

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Frieda

June 8, 2017

How do you know which one to choose to unmount? After typing diskutil list there are 14 dev/disk? Trying to erase hard drive and had “can’t unmount” error.

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1. Alan Siu
  
  June 8, 2017
  
  Try typing mount to see what is mounted where.
  
  Reply
Scott

July 16, 2017

Thanks! Worked like a charm 🙂

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Andy

August 6, 2017

Hi Alan,

Trying to erase my hard drive and it won’t unmount: Disk erase failed. Couldn’t ummount disk.

The diskutil list command brought up just /dev/disk0 (plus disk0s1, disk0s2 and disk0s3). The command “sudo diskutil unmountDisk force /devdisk0” failed because “at least one volume could not be unmounted.”

I followed your further guidance below to another blog and the command “hdiutil detach /dev/disk0” resulted in the following message: “hdiutil: couldn’t unmount ‘disk0’ – Resource busy.”

I followed your additional further guidance below and see the command “sudo lsof | grep /Volumes/myDrive” – what is supposed to be substituted in for “myDrive”? I’ve tried many things and everything simple ends up resetting the Terminal for a new command query.

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1. Alan Siu
  
  August 7, 2017
  
  /dev/disk# is where the drive is physically, and /Volumes/NAMEOFDRIVE is where you can actually find the drive in Finder.
  
  Three quick ways to find that:
  
  1. Type /Volumes/ and then hit the Tab key on your keyboard twice to see what the options are.
  
  2. Type mount and hit Enter to see what paths things are mounted to.
  
  3. Type df -h and hit Enter to see free disk space (but this will also show the mounted paths).
  
  Reply
  1. Andy
    
    September 4, 2017
    
    1. Typing “/Volumes/” and hitting Tab once yielded “Macintosh HD”; Tab a second time did nothing.
    
    2. Typing “mount” and hitting Enter yielded:
    
    /dev/disk0s2 on / (hfs, local, journaled)
    devfs on /dev (devfs, local, nobrowse)
    map -hosts on /net (autofs, nosuid, automounted, nobrowse)
    map auto_home on /home (autofs, automounted, nobrowse)
    
    3. Typing “df -h” yielded:
    
    Filesystem; Size; Used; Avail; Capacity; iused; ifree; %iused; Mounted on
    
    /dev/disk0s2; 297Gi; 9.1Gi; 288Gi; 4%; 2455523; 75477379; 3%; /
    devfs; 181Ki; 181Ki; 0Bi; 100%; 626; 0; 100%; /dev
    map -hosts; 0Bi; 0Bi; 0Bi; 100%; 0; 0; 100%; /net
    map auto_home; 0Bi; 0Bi; 0Bi; 100%; 0; 0; 100%; /home
    
    Thank you for the additional assistance but I’m not quite sure what to do with any of this. I tried the command “$ sudo lsof | grep /Volume/Macintosh\ HD” but it resulted in nothing, I just got a reset terminal after it prompted me for my password. I also tried it in a different command on the other page you referenced: typing “$ sudo diskutil unmount force /Volumes/Macintosh\ HD” resulted in “Volume Macintosh HD on disk0s2 failed to unmount.”
    
    Do you have any other troubleshooting tips?
    
    Reply
    1. Alan Siu
      
      September 4, 2017
      
      Why are you trying to unmount Macintosh HD? Are you booting from an external drive? If you’re not booting from an external drive, Macintosh HD is the drive you’re using when you boot up your Mac. Based on your mount and df -h output, it doesn’t seem you have anything mounted that needs to be unmounted.
      
      Reply
      1. Andy
        
        September 4, 2017
        
        I am not using an external drive, I’m trying to erase my startup disk before discarding my old iMac. The Apple instructions (https://support.apple.com/en-us/HT204904) state to select the disk, not the volume, in Disk Utility but when I try to do so, I get the following error message: “Disk erase failed. Couldn’t ummount disk” mentioned in my first comment above and which I thought this article was about troubleshooting around.
      2. Alan Siu
        
        September 4, 2017
        
        You won’t be able to erase the drive while you’re using it. Boot into recovery mode (hold down Cmd-R while rebooting) and then you should be able to unmount and erase Macintosh HD.
Andy

September 30, 2017

Don’t I need to erase the disk rather than the volume? That’s what the Mac help page says regarding wiping a computer to sell or donate. I can successfully erase Macintosh HD (the only partition) but I get the error message “Disk Erase Failed: couldn’t unmount disk” when I try to erase the disk itself. Hence this series if inquiries.

Reply
1. Alan Siu
  
  September 30, 2017
  
  There’s nothing user-specific on your recovery partition if you wipe the Macintosh HD partition. If you really want to erase the entire disk, including the recovery partition, you must boot from an external drive.
  
  Reply
  1. Andy
    
    September 30, 2017
    
    It’s funny, now that I check the Apple help page again, it’s been revised as of four days ago and the note that used to be included (make sure to select the disk, not the volume) is now gone – figures! Anyway, it sounds like I should be good to go. Thanks for the help, I really appreciate it.
    
    Reply
Lara

December 14, 2017

Thank you so much! It took me an hour to find your page and I am glad that I did. You have made my day!

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Ronald

January 11, 2018

started with above suggestions and part of it worked. Wanted to format it but then it still says it needs an unmounted disk.

at first the disk looks like this:

/dev/disk4 (external, physical):
#: TYPE NAME SIZE IDENTIFIER
0: FDisk_partition_scheme *16.0 GB disk4
1: Windows_FAT_32 boot 66.1 MB disk4s1
2: Linux 16.0 GB disk4s2

Then i forced the disk to unmount:

iMac-van-Ronald:~ koenie$ diskutil unmountDisk force /dev/disk4
Forced unmount of all volumes on disk4 was successful

Formatting:

iMac-van-Ronald:~ koenie$ sudo diskutil eraseDisk FAT32 ROLF MBRFormat /dev/disk4
Password:
Started erase on disk4
Unmounting disk
Creating the partition map
Waiting for partitions to activate
Formatting disk4s1 as MS-DOS (FAT32) with name ROLF
Error: -69830: This operation requires an unmounted disk

what to do, besides throwing it out off the window…….

Reply
1. larson
  
  January 7, 2019
  
  Hi Ronald, I ran across your post and see you had the same issue im having now with no luck in finding a solution. This was a year ago I realize but do you happen to remember how you fixed your SDcard?
  
  Reply
Leslie

February 27, 2018

HUGE help! Simple solution that worked perfectly, and all the other solutions I found were very complex and would have been time consuming. Thank you so much.

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Joy

April 1, 2018

Hello,
I have the same problem and I ran into you website. I was trying to erase an external hard drive, and got this error. Following your directions, the disk was unmounted successfully in terminal. At Disk Utility, I saw the partitions of the external hard drive being greyed out. So I clicked at the hard drive and clicked Erase, but still got the same error. After ejecting it and reconnecting the drive, it seemed that nothing has been erased. What would you suggest? Thanks!

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michelle

September 18, 2018

this is the closest i’ve gotten to be able to erase this external disk, but… i got this error message:
Error: -69825: Wiping volume data to prevent future accidental probing failed

Reply
1. Alan Siu
  
  September 18, 2018
  
  When I Googled your error, this came up as a solution:
  https://www.ifixit.com/Answers/View/244571/partition+failed+wiping+volume+data+to+prevent+future+accidental+probi
  
  Reply
MRDisperatti

December 10, 2018

Tried but…

sudo diskutil unmountDisk force /dev/disk2
sudo: unable to stat /etc/sudoers: Permission denied
sudo: no valid sudoers sources found, quitting

Reply
1. Alan Siu
  
  December 11, 2018
  
  You’re not a valid sudoer, apparently. Are you running those commands as an admin user?
  
  Reply
A

February 8, 2019

THANK YOU!!!!!!

Reply
Jeff

May 8, 2019

Thank you, Alan! Your procedure worked perfectly — you got me out of a very frustrating situation.

Reply