This errno, "Directory not empty", occurs principally if a rmdir(2) operation is called to remove a directory which is not "empty" (that is, contains more than the standard dirents, "." and ".."). It may also occur if you try to rename(2) a directory onto a non-empty target directory, or now in the sysctl subsystem if a non-recursive attempt is made to remove a non-empty OID from the MIB.