zie ook: http://support.apple.com/kb/HT2426 (AirPort Extreme (802.11n): USB storage device supported formats and protocols)
The AirPort Extreme (802.11n) supports USB storage devices that have a block size of 512 bytes, and are formatted as Mac OS Extended (HFS-plus), FAT16, or FAT32. Not all USB storage devices use a block size of 512 bytes.
The AirPort Extreme (802.11n) shares storage devices based on the format used to initialize the storage device. For example, if HFS-plus formatting was used, AFP and SMB/CIFS protocols are used to share the device on the network. If FAT16 or FAT32 was used, SMB/CIFS protocols are used.
The AirPort Extreme (802.11n) works with disks that have a single partition and 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 is supported.
Note: Use AirPort Disk Utility to discover and mount AirPort Extreme-based volumes over the network.
How many USB 2.0 hard drives and printers can be connected?
The USB 2.0 specification states you can use up to 127 USB devices. One of those “devices” is the USB port itself, so you could potentially add an additional 126 devices to the AirPort Extreme (802.11n) Base Station.
These devices must follow the USB 2.0 power specification. For more information, see (http://www.usb.org/developers/docs/).