Other Unmount samba share shows "failed: Device busy" error

I've a samba mount (//DOKWII@MAILHOST03/MAIL 1.5T 569G 1.0T 36% /storage/mailbacks) that I am trying to unmount. I use umount /storage/mailbacks and it says
umount: unmount of /storage/mailbacks failed: Device busy

Apparently it could be because I've a shell opened at that directory, but my current pwd isn't /storage/mailbacks. Then I read that I could find open files using lsof(8) or fstat -f /storage/mailbacks, but these commands still didn't output anything. I've also tried using sudo umount -at smbfs, still nothing.

I've read that I could use umount -f /storage/mailbacks to force unmounting. However, am a little caution about this option as am not sure if it could cause data loss or server crashing (I had an experience where the server couldn't boot because it couldn't mount the samba share when I added it on /etc/fstab).

What should I do? thanks
I had to reboot the server, which again had to do some system file checks. It took another 3 hours to get back online.