man umount -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.)
最近利用 NFS 傳送一些大檔案，但是總會遇到一兩個卡住系統，導致整個系統崩壞，需要 umount 但卻出現 Device is busy 導致無法解除 mount 因此必須利用 umount -l /mount/path/