Umount target is busy

“(In some cases useful info about processes that
use the device is found by lsof(8) or fuser(1).)”

As there are no lsof or fuser, what is the way to go for finding out who is holding the resource?

sudo apt-get install psmisc

Then you can use fuser.

You could install lsof or fuser.

Do you have samba installed ? If an SMB client is still connected (explorer windows open to a share for example) then it will lock the drive and prevent it unmounting.

In that case make sure no clients are using shared files then try unmounting again after about a minute.

I have restarted samba and than I was able to umount the drive.
I believe that I will install psmisc as mcobit suggested for the future diagnostics.

If restarting samba allowed you to unmount the drive it means some client on your network connected to the samba share and was still connected. Usually closing any programs (and explorer windows) on the clients that may be accessing the share and then waiting about 30 seconds to a minute is enough for the lock on the drive to be removed allowing it to be unmounted.

I have occasionally seem times where this did not work though, and suspect it might be caused by something like a virus scanner accessing the share in the background even though you have closed visible programs that were using the share. This is particularly likely if you have a drive letter mapped to the share.

I have closed all visible programs that were using the samba but still I wasn’t able to umount the drive.
How come that losf and fuser are not included in osmc as defaults?

You can install them whenever you want but I think everything that is not necessary for the system to function was cut to get the image size down.

If you are sure that nothing is using the share anymore you can fuser -k mountpoint to kill preocesses locking it up. Then you can umount.