If your base station supports connecting to a USB hard disk, or you’re using a Time Capsule with an internal AirPort disk, you can secure access to the disk. You can secure the hard disk either on a per-user basis if you have set up user accounts on your base station, or by assigning a password to the disk.
Open AirPort Utility, select your base station or Time Capsule, and then choose Base Station > Manual Setup. Enter the device password if necessary. Click Disks.
AirPort Utility is located in the Utilities folder in the Applications folder on a computer using Mac OS X, and in Start > All Programs > AirPort on a Windows computer.
Choose "With Accounts" or "With a single password" from the Secure Shared Disks pop-up menu.
If you choose "With a single password," enter a password. Users will need to enter this password to access the disk.
If you choose "With Accounts," users will need to authenticate with the user name and password you set up for them in the accounts pane of AirPort Utility. For information about setting up user accounts, click the link below.
You can also control guest access to the disk, allowing guests to have read only, read and write access, or no access.
If you change the way the disk is secured (from user accounts to the device or disk password) or if you change the name of an account, users may not have access to all the files on the disk.