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My question is quite simple, if I am to enter these two line in OS X Terminal, what should happen?

xupengjunde-iMac:Shane Shane$ cd /Volumes/External
xupengjunde-iMac:External Shane$ sudo chmod -RN *

The R flag on chmod means recursive, changing permission for every file/folder. The N flag means to remove ACL on files.

Now what about *? Will that mean all files on /Volumes/External or all files on my Mac?

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Bash expands * to the list of all files and directories in the current directory, except for the hidden ones (those starting with .). See TLDP for the full explanation.

To answer your specific question: The command will change permissions on all files and directories inside /Volumes/External recursively. This means that any filesystems that are mounted in this directory will be changed recursively as well.

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How is chmod rm... It will remove all ACL on those files... – Shane Hsu Jul 12 '13 at 12:50
Of course you're right, somehow I read rm instead of chmod... – Jannik Jochem Jul 12 '13 at 15:52

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