How do I forcefully unmount a Linux disk partition?

by on January 27, 2006 · 10 comments· LAST UPDATED December 21, 2007

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It happens many times you try to unmount a disk partition or mounted CD/DVD disk and if you try to unmount device, which is accessed by other users, then you will get error umount: /xxx: device is busy. However, Linux/FreeBSD comes with fuser command to kill forcefully mounted partition.


Understanding device error busy error

What happens basically, is that Linux / UNIX will not allow you to unmount a device that is busy. There are many reasons for this (such as program accessing partition or open file) , but the most important one is to prevent data loss.

Try the following command to find out what processes have activities on the device/partition. If your device name is /dev/sdb1, enter the following command as root user:
# lsof | grep '/dev/sda1'
Output:

vi 4453       vivek    3u      BLK        8,1                 8167 /dev/sda1

Above output tells that user vivek has a vi process running that is using /dev/sda1. All you have to do is stop vi process and run umount again. As soon as that program terminates its task, the device will no longer be busy and you can unmount it with the following command:
# umount /dev/sda1

Following disussion allows you to unmout device and partition forcefully using Linux commands.

Linux fuser command to forcefully unmount a disk partition

Suppose you have /dev/sda1 mounted on /mnt directory then you can use fuser command as follows:

WARNING! These examples may result into data loss if not executed properly (see "Understanding device error busy error" for more information).

Type the command to unmount /mnt forcefully:
# fuser -km /mnt
Where,

  • -k : Kill processes accessing the file.
  • -m : Name specifies a file on a mounted file system or a block device that is mounted. In above example you are using /mnt

Linux umount command to unmount a disk partition
You can also try umount command with –l option:
# umount -l /mnt
Where,

  • -l : Also known as Lazy unmount. Detach the filesystem from the filesystem hierarchy now, and cleanup all references to the filesystem as soon as it is not busy anymore. This option works with kernel version 2.4.11+ and above only.

If you would like to unmount a NFS mount point then try following command:
# umount -f /mnt
Where,

  • -f: Force unmount in case of an unreachable NFS system

Caution: Using these commands or option can cause data loss for open files; programs which access files after the file system has been unmounted will get an error.

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{ 10 comments… read them below or add one }

1 Lukasz January 30, 2009 at 1:40 pm

Very useful, thank you.

Reply

2 mohammed a.rahman June 20, 2009 at 6:13 pm

thanks

Reply

3 Anil Kumar Jena April 29, 2010 at 8:30 pm

fuser -km {file / filesystem}
If you have NFS mount file then it will now work.

Reply

4 Mark November 18, 2010 at 10:28 pm

Sometimes fuser and lsof fail. One option that works for me is remounting read-only, and then doing a lazy unmount:

mount -o ro,remount /dev/sdb3
umount -l /dev/sdb3

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5 Christian February 24, 2011 at 8:57 am

I have a cronjob that needs to mount an external hdd and then unmount after it backup to my external hdd.

This is how my backup.pl inside my cron.daily looks like:-

system (“/etc/rc.d/init.d/sendmail stop”);
system (“/usr/sbin/cmuExport -d”);
system (“/etc/rc.d/init.d/sendmail start”);

system (“mkdir /mnt/usbdisk”);
system (“mount -t ext3 /dev/sdb1/mnt/usbdisk”);
system (“/bin/tar -zcvf /home/cmu/localhost.tar.gz/home/cmu/localhost”);
system (“mv /home/cmu/localhost.tar.gz/mnt/usbdisk”);
system (“umount -l /dev/sdb1/mnt/usbdisk”);

But it doesn’t unmount it. The error message says:-

umount: /mnt/usbdisk: not mounted
umount: /mnt/usbdisk: not mounted

I need help, please thank you.

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6 VItaly March 20, 2011 at 8:44 pm

First: Why are you using perl script instead of bash script if all calls are system’s? You can just delete “system” before all of your strings and rename backup.pl to backup.sh. At least this will be easier to debug.

Second: You have an error in umount command. Here command you needed:
“umount -l /dev/sdb1″.
In fact your command must work for your /dev/sdb1. Error was coz system trying to find block device “/mnt/usbdisk”, but this is a mount point.

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7 Zahid Haseeb March 19, 2012 at 5:58 am

@ nixcraft

As you said
# lsof | grep ‘/dev/sda1′

what about if we have a cifs mounted partition ?

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8 naildownx August 6, 2013 at 7:15 pm

the fuser -km /mnt/SHARE command worked for me! Thanks!

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9 Tim Macking November 6, 2013 at 7:01 pm

This will work if you have NFS mount fuser -km {file / filesystem}

——————
Tim Macking

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10 Gaurav Kumar Garg March 24, 2014 at 7:32 am

for forcefully unmounting use command

umount -l /your_mount_path

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