You can connect external USB storage devices to the following AirPort base stations:

• AirPort Extreme 802.11n (1st Generation)
• AirPort Extreme 802.11n (2nd Generation)
• AirPort Extreme 802.11n (3rd Generation)
• AirPort Extreme 802.11n (4th Generation)
• AirPort Time Capsule 802.11n (1st Generation)
• AirPort Time Capsule 802.11n (2nd Generation)
• AirPort Time Capsule 802.11n (3rd Generation)
• AirPort Time Capsule 802.11ac
• AirPort Extreme 802.11ac

Supported formats:

• Mac OS Extended (HFS-plus) - Formatting a storage device as Mac OS Extended with journaling is recommended, because it may provide additional resilience if the device is removed or powered down while in use.
• FAT16
• FAT32

Note: ExFAT and NTFS formats aren't supported.

Formatting AirPort base station USB disks

To format a USB disk for HFS-plus or Fat 16/32, connect it directly to a computer and use the computer's own formatting application (For OS X, it's called Disk Utility). When formatting is completed, the disk is ready to be connected to your AirPort base station.

External USB storage devices may come to you preformatted as HFS+ or FAT16/32. Before connecting a USB disk to your AirPort base station, you may want to reformat them as HFS-plus if you want to use them with Apple Filing Protocol (AFP) and Bonjour, so that they can be more easily identified and shared on the network. 

RAID Support

AirPort base stations work with most disks that are not software RAID volumes (no more than one volume per physical disk). If the disk is a self-contained RAID that presents itself to a computer as a single volume requiring no software support, then it may be supported. Consult the storage manufacturer for compatability.

USB Power

Some USB disks draw power from the USB port, while others have their own power supply. In order to be powered by AirPort base stations, USB disks without their own power supply must follow the USB 2.0 power specification; for more information, see http://www.usb.org/developers/docs/. If your storage device has its own power supply, we recommend you use it.

Network Disk Sharing

If HFS+ formatting is used, AFP and Microsoft SMB/CIFS protocols are used to share the device on the network. If a FAT16/32 format is used, only SMB/CIFS protocols are used and AFP won't be available.

Note: Use AirPort Disk Utility to discover and mount connected USB storage device volumes over the network in Mac OS X v10.4 or earlier. In Mac OS X v10.5 or later, the "SHARED" item in the sidebar will show the volumes if enabled to do so.  To enable this item, follow these steps:

1. From the Desktop, click Finder > Preferences.
2. Click Sidebar.
3. Under DEVICES, check External disks.

USB port specification

The USB port on AirPort base stations subscribes to the USB 2.0 specification, and supports the same number of devices as described in http://www.usb.org/developers/docs/.

Time Machine support for USB disks

OS X Time Machine supports compatible USB disks connected to AirPort Time Capsule (802.11n and 802.11ac), and AirPort Extreme (802.11ac).