A preliminary manual of AirPort Express reveals that Apple had integrated the ability to exploit hard drives connected to the USB port, a function evidently eliminated at the last moment by Apple.
Apple’s AirPort Express is a convenient 802.11n Wi-Fi hotspot with simultaneous dual-band support. Apple AirPort Express setup can be used to extend the reach of an existing network, provide AirPlay functionality, print wirelessly using a printer connected to the USB port (when connecting a USB printer to AirPort Express, all computers on the wireless network can use it to print). There are many benefits of having AirPort Express at home, as it can work as a router and deliver fast and seamless internet service to the users, but also act a medium to connect to other devices for printing purpose.
The USB port of the device in question cannot be used in combination with a self-powered USB hard drive; this last function is only supported by AirPort Time Capsule 802.11ac, AirPort Time Capsule 802.11n (from 1st to 4th generation), AirPort Extreme 802.11ac and AirPort Extreme 802.11n (1st to 5th generation).
If the hard drive has an external power source, only then will it be able to operate with Apple AirPort. This could perhaps be the reason why Apple has taken a U-turn when it comes to adding the USB port. However, there are many other tasks that AirPort Express is proficient in doing. You will only be able to identify when you begin using the device.
On the AirPort Express setup 802.11n (2nd generation) and AirPort Express 802.11n (1st generation) the USB port can only be used for connecting a printer. One of the top news company has unearthed a preliminary version of the manual of the base and in this, it was reported the possibility of connecting a USB disk.
This function was, therefore, deliberately eliminated at the last moment. Apple may have encountered some problems and preferred to reserve this feature in future versions of the device. Well, AirPort Express is a product by Apple and they reserve the right to add or eliminate any feature in the device. Therefore, there is nothing much that users can do except waiting until Apple adds it again.