If you connect a USB hard disk to your AirPort Extreme Base Station, computers connected to the network, both wireless and wired, can use it to back up, store, and share files.
To share a USB hard disk on you network:
1 Plug the hard disk into the USB port on the back of the base station.
2 Open AirPort Utility, located in the Utilities folder in the Applications folder on a Mac, and in Start > All Programs > AirPort on a computer using Windows
3 Select your base station, and then choose Manual Setup from the Base Station menu, or double-click the base station to open its configuration in a separate window. Enter the base station password if necessary.
4 Click Disks in the toolbar, and then click File Sharing.
5 Choose “With a disk password,” or “With base station password” if you want to secure the shared disk with a password, or choose “With accounts” if you want to secure the disk using accounts.
If you choose to use accounts, click Configure Accounts, click the Add (+) button, and then enter a name and password for each user that will access the disk.
6 Choose “Not allowed,” “Read only,” or “Read and write” to assign guest access to the disk.
7 Select the “Share disks over Ethernet WAN port” checkbox if you want to provide remote access to the disk over the WAN port.