1. 你的某个 console 的当前目录正在该 mount 点的目录或子目录;
2. 某个程序可能正在使用着该 mount 点下面的某个文件;
总之是某个(或几个)程序正在使用你mount上的设备,mount 的提示很明白。关于这一点,你可以在 man umount 等文档中看到,有详细的说明,这是 man 中的一段:
Note that a file system cannot be unmounted when it is `busy' - for example, when there
are open files on it, or when some process has its working directory there, or when a
swap file on it is in use. The offending process could even be umount itself - it opens
libc, and libc in its turn may open for example locale files.