In very basic terms, you have your NAS central in your network.
Once the NAS has been set up, you enter the user manager and create users for each person on your home network.
You can then enable DLNA - this will create 3 shared folder - music, video, and photos. You can give users read or read/write to this, and they can use them as their shared folders. By using DLNA you can access the media in these shared folder on consoles, smart phones, and much more.
You can create additional shared folders if you wish, or you can leave DLNA altogether and create your own shared folders - assigning persmissions to each user as need be.
As for private areas - create a folder for each user. Give each user account permission to their folder.
On each user's PC/MAC simply map a drive to the folder you have created for them, using their own user credentials to connect.
1. Users have access to shared folders.
2. Users have access to their own private folder.
3. Users do not even see any other private user folders.
When it comes to backup, you can schedule a backup to an external drive, or to a DX510.
You may find some of these videos useful:
Backup to an external drive:http://www.youtube.com/watch?v=K1Ya5nKyfDQ&safe=active
Setting up and backups to a DX510 expansion unit:http://www.youtube.com/watch?v=B6nc6GKU_8E&safe=active