How to force mdadm to stop RAID5 array?

I realize that this is an old question and the original poster believed that SAMBA was the issue, but I experienced the same exact problem and think that very likely the issue was not SAMBA (I actually don’t even have SAMBA), since it didn’t show up in the lsof output, but rather the user was already in the RAID mount-point directory when they switched to root or did a sudo.

In my case, the problem was that I started my root shell when my regular user was in a directory located on that mounted /dev/md127 drive.

user1@comp1:/mnt/md127_content/something$ su -
root@comp1:~# umount /dev/md127
umount: /dev/md127: target is busy

Here is the output of lsof in my case:

root@comp1:root@comp1:~# lsof | grep /dev/md127
md127_rai  145            root  cwd       DIR      253,0     4096          2 /
md127_rai  145            root  rtd       DIR      253,0     4096          2 /
md127_rai  145            root  txt   unknown                                /proc/145/exe

Even though lsof | grep md125 didn’t show any processes except [md127_raid1], I could not unmount /dev/md127. And while umount -l /dev/md127 does hide /dev/md127 from the output of mount, the drive is apparently still busy, and when mdadm --stop /dev/md127 is attempted, the same error is shown:

mdadm: Cannot get exclusive access to /dev/md127:Perhaps a running process, mounted filesystem or active volume group?

SOLUTION is simple: check if there are any users logged in who are still in a directory on that drive. Especially, check if the root shell you are using was started when your regular user's current directory was on that drive. Switch to that users shell (maybe just exit your root shall), move somewhere else, and umount and mdadm --stop will work:

root@comp1:~# exit
user1@comp1:/mnt/md127_content/something$ cd /
user1@comp1:/$ su -
root@comp1:~# umount /dev/md127
root@comp1:~# mdadm --stop /dev/md127
mdadm: stopped /dev/md127

If you're using LVM on top of mdadm, sometimes LVM will not delete the Device Mapper devices when deactivating the volume group. You can delete it manually.

1. Ensure there's nothing in the output of sudo vgdisplay.
2. Look in /dev/mapper/. Aside from the control file, there should be a Device Mapper device named after your volume group, e.g. VolGroupArray-name.
3. Run sudo dmsetup remove VolGroupArray-name (substituting VolGroupArray-name with the name of the Device Mapper device).
4. You should now be able to run sudo mdadm --stop /dev/md0 (or whatever the name of the mdadm device is).

I was running into similar issues but I didn't have the raid device mounted in any way. Stopping SAMBA didn't seem to help either. lsof showed nothing.

Everything just resulted in:

# mdadm --stop /dev/md2
mdadm: Cannot get exclusive access to /dev/md2:Perhaps a running process, mounted filesystem or active volume group?

What finally fixed it for me was remembering that this was a swap partition - so I just had to swapoff /dev/md2 - this allowed me to mdadm --stop /dev/md2 successfully.