Airport Extreme: Macbooks cannot connect to disk drive over wifi; LAN iMac can Just bought an Airport Extreme. Connected a LaCie 2To hddrive (usb 2).

I have set up the drive without issue and can indeed see it in the Finder  and browse through the files via my iMac (which connects via LAN).

But on the two macbooks over Wifi, I can see the Airport Extreme in the Finder, and connect to the internet without problem. Everything seems ok, except that the HDD does not appear.

Any idea why?
On my MacBooks over Wifi, the HDD attached to the Airport does not mount in the same way as directly connected or even NAS drives. Rather than the drive, I get the Airport Extreme in the 'Shared' section of Finder. Selecting the Airport Extreme shows the drive as a folder, and opening that folder effectively 'mounts' it. Even then, the drive does not show as a drive within Shared or Devices, yet it is accessible.