umount -l
有时候，umount一个分区，提示设备忙，加上 -f 参数也不管用，然后 lsof 路径 又找不到任何进程，这时候就可以用 sudo umount -l 路径 卸载之。
man 如是说：
-f Force unmount (in case of an unreachable NFS system). (Requires kernel 2.1.116 or later.)
-l Lazy unmount. Detach the filesystem from the filesystem hierarchy now, and cleanup all references to the filesystem as soon as it is not busy anymore. (Requires kernel 2.4.11 or later.)