Hi,
First to deal with the popup window, the next time you have this popup, leave it as is and go to "Control Panel" (use small icons) and then open "Credentials Manager". Go down and click on the "Add a Windows Credential" and then enter your log in info and close it and the original popup. Then try going back into the 320. It should open fine now and will be remembered.
I am not sure how you and your users are accessing your 320, from the same network or via ftp, but it should not be too difficult to set up 2 network shares, Volume_1 and Volume_2. For yourself, I would give read and write access to yourself, otherwise you will have to log on to the admin account to access both volumes. Then for the other users, give them whatever access level you want to "their" volume and set "deny" access to the Volume you want to keep for yourself. I am not sure about the USB Volume as I don't use one but I think there is a problem blocking that and or setting it up via network shares. Maybe someone else can help you or do a little trial and error. Hope this helps. Good luck.