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I've set the permissions on files on my removable external drives in ways that seem to make sense to preserve some degree of security. For example, for files that only I should have access to, I've only myself any access rights

-rw-------@   1 MyName  staff  ... File1 

while for others, that I want other users of my machine to have access to, I've expanded these access rights a bit

-rw-r-----@   1 MyName  staff  ... File2

This behaves as I intend on my machine; but what happens if the drive is plugged into a different machine? Does anyone on the staff group there have read access to File2 and none to File? Would anyone who was given the user name MyName on that machine have full access to both files?

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up vote 2 down vote accepted

Anyone with the same user ID as you have on your main machine will be able to read the disk. The, usually hidden, user Ids start at 501 and increase. If you are not the first user on a machine you will not be able to read the contents of the drive.

I've gone round and round with this. If you need a secure drive then you need to encrypt it with a password. There are ways to do this using the tools built into Mountain Lion, or you can use a third party solution like True Crypt.

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So its not the name, but the ID that will give access. Correct? For example, on another Mac, where staff has the same ID by default, all members of staff there will have the same access they have on my machine, while any user there who happens to have my user ID, regardless of what their user name is, will have the same access I have. – raxacoricofallapatorius Feb 5 '13 at 19:45
That is correct. It's a lousy way of doing things, but it's what we've got. – Mark Feb 5 '13 at 19:54
Yes, I've had to do other things to secure the drives, but was wondering if there was anything useful that the permissions were doing to. Clearly not. Presumably the same is true for network access? – raxacoricofallapatorius Feb 5 '13 at 19:56
Yeah, it's all based on the same 60's version of privacy. – Mark Feb 5 '13 at 20:03

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