If you use the "Main/Admin" username and password, you should get the entire volume to mount. Other wise, added users will only get their "Home" directory and any public share you may have set up... which by default is the "Public Folder" ... If your trying to make every user account on the system act as an administative account you need to select "Allow user to Administer this computer" via the Users control panel. This is not what you would normally do . This is why you would have a public share and each user has their own home directoy...
As far as file permissions with-in a public share, the Unix'ish way is to set up groups and set group permissions... Un-like NT where you can have tons of users with each having different permissions for just that file set via a permissions button... the Unix'ish method is owner-user-group where you have an individual user permission (the Owner persay) then you have a group permission ... this is where you would need to add different users to that group.. you can accomplish a similar effect as with NT, it's just different and you have to plan it out vs just click and add a user to that file. You have to plan and create different groups and add users to those groups. I don't know how to explain this stuff very well...
But I do how ever understand the frustration of it all when your first learning how to do all this on something other than NT. I must say, that is one thing NT got right, minus the security issue, it is nice to simply right click a file or folder and add tons or just two users with different permissions for each or the same for all or even do the group thing.... at any rate It is easier for the not technical... OS X is well... Unix'ish ;-)
