umount -lf /mnt 强制卸载文件
-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.)
原文：http://www.cnblogs.com/gaoyuechen/p/7528613.html