Can an Apple TV use media files on an Airport Disk?
You can't use an Airport Disk directly with Apple TV
It should be noted that 40GB is a lot of space. I remember the days when I thought a 1GB was a lot of space. However when it comes to digital video, 40GB is certainly on the small side. My own digital video collection already fills a 1TB external drive.
The new Apple TV comes with a 40GB drive which syncs with your Macs.
The new Apple 802.11n Airport Extreme Base Station now comes with the added functionality of support for external USB 2 hard drives, and using an USB hub you can add multiple drives. This is referred to by Apple as Airport Disk.
It would appear that it would be nice if the Apple TV could access media files on your Airport Disk (or Disks) which would mean you could have a really large media library and wouldn't need to leave a Mac on.
However according to a blog report from O'Reilly, Apple has confirmed that AirPort disks are not available to Apple TV.
You can have your iTunes library on an external disk and this may be possible to use an Airport Disk for this, but to use this library with an Apple TV, you would need to leave your Mac on, if it was turned off, or went to sleep then the Apple TV would no longer be able to
So this means that this idea is not an option.