求高手帮我看下关于 mv命令为什么出现 Directory not empty提示错误！谢谢！！

354779608fwfw

实验平台：VM7.0 上运行Ubuntu 11

实验代码：
root@ubuntu:/usr/local/arm# ls
4.2.2-eabi
root@ubuntu:/usr/local/arm# cd ..
root@ubuntu:/usr/local# ls
4.2.2-eabi  arm  bin  etc  games  include  lib  man  sbin  share  src
root@ubuntu:/usr/local# mv -i 4.2.2-eabi /usr/local/arm
mv: overwrite `/usr/local/arm/4.2.2-eabi'? y
mv: cannot move `4.2.2-eabi' to `/usr/local/arm/4.2.2-eabi': Directory not empty
root@ubuntu:/usr/local# mv  4.2.2-eabi /usr/local/arm
mv: cannot move `4.2.2-eabi' to `/usr/local/arm/4.2.2-eabi': Directory not empty
root@ubuntu:/usr/local# mv -f 4.2.2-eabi /usr/local/arm
mv: cannot move `4.2.2-eabi' to `/usr/local/arm/4.2.2-eabi': Directory not empty
root@ubuntu:/usr/local#
就想问下，为什么mv不了，出现‘mv: cannot move `4.2.2-eabi' to `/usr/local/arm/4.2.2-eabi': Directory not empty
’提示，其中，我可以用cp -r 来实现，此外，/usr/local/arm和/usr/local的权限都设置为为777，最大。麻烦高手请教！谢谢！！！