What is the effect of permissions on external drives? 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?
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.