Not able to unmount a Nas folder, target busy

Question

I have some issues to unmount my Nas folder. When I try to unmount it with sudo umount /media/Nas6To i get a message telling me that the device is busy, and I cannot unmount it:

thomas@NZXT:~$ sudo umount /media/Nas6To 
umount: /media/Nas6To: the target is active.

I have found on different post using fuser and lsof to identify the process keeping it busy. Unfortunately those commands returns nothing.

thomas@NZXT:~$ fuser -m /media/Nas6To
thomas@NZXT:~$

thomas@NZXT:~$ lsof | grep Nas6To
thomas@NZXT:~$

I would like to undestand what is keeping it busy. And what I could do be able to unmount it with sudo umount /media/Nas6To

My configuration:

Fstab configuration:

//10.0.0.110/6To     /media/Nas6To cifs username=xxxx,password=xxxx,uid=1000,vers=2.0,iocharset=utf8,file_mode=0775,dir_mode=0770,noauto 0 0

Directory permissions (when mounted):

thomas@NZXT:~$ ls -la /media
total 28
drwxr-xr-x   8 root    root 4096 feb.  28 14:40 .
drwxr-xr-x  20 root    root 4096 jan.  30 15:25 ..
drwxrwx---   2 thomas  root    0 mars  13 13:59 Nas6To

OS (Xubuntu):

thomas@NZXT:~$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 20.04.2 LTS
Release:    20.04
Codename:   focal

Answer 1

Hi after some time I found the fix using wireshark.

I was not able to unmount because my computer was sending lot of SMB packet related to the trash on background.

Note: The following commands will empty your Trash. Before continuing, check that it doesn't contains any thing that you want to keep.

To fix it:

• Delete your Nas Trash with rm -r /PathToYourNAS/.Trash-1000/
• Flush your computers Trash with rm -r ~/.local/share/Trash/*