Occasionally, we’ll see filesystems, such as NFS, that are busy or hung cannot be unmounted. Here are some things to try.
Check for processes accessing the filesystem.
$ fuser -uv /my_filesystem
Another way to find which processes are holding open files.
$ lsof | grep /my_filesystem
Kill all processes accessing filesystem
$ fuser -km /my_filesystem
List all files held open by processes, with link counts of 0.
$ lsof +L 1
Try a lazy unmount
$ umount -l /my_filesystem